doc: Convert semihosting readme to rST

This converts the semihosting readme to rST. I have tried to make only
cosmetic changes, but I did fix up the first link (which was broken).
Signed-off-by: NSean Anderson <sean.anderson@seco.com>

doc/usage/index.rst

@@ -11,6 +11,7 @@ Use U-Boot
    netconsole
    partitions
    cmdline
+   semihosting
 
 Shell commands
 --------------

doc/README.semihosting → doc/usage/semihosting.rst

-SPDX-License-Identifier: GPL-2.0+
-/*
- * Copyright 2014 Broadcom Corporation.
- */
+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright 2014 Broadcom Corporation.
+
+Semihosting
+===========
 
 Semihosting is ARM's way of having a real or virtual target communicate
 with a host or host debugger for basic operations such as file I/O,
-console I/O, etc. Please see
-http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0471c/Bgbjjgij.html for more information.
+console I/O, etc. Please see `Arm's semihosting documentation
+<https://developer.arm.com/documentation/100863/latest/>`_ for more
+information.
 
 For developing on armv8 virtual fastmodel platforms, semihosting is a
 valuable tool since it allows access to image/configuration files before
 eMMC or other NV media are available.
 
 There are two main ARM virtual Fixed Virtual Platform (FVP) models,
-Versatile Express (VE) FVP and BASE FVP (See
-http://www.arm.com/products/tools/models/fast-models/foundation-model.php)
+`Versatile Express (VE) FVP and BASE FVP
+<http://www.arm.com/products/tools/models/fast-models/foundation-model.php>`_.
 The initial vexpress64 u-boot board created here runs on the VE virtual
 platform using the license-free Foundation_v8 simulator. Fortunately,
 the Foundation_v8 simulator also supports the BASE_FVP model which
 companies can purchase licenses for and contain much more functionality.
-So we can, in u-boot, run either model by either using the VE FVP (default),
-or turning on CONFIG_BASE_FVP for the more full featured model.
+So we can, in U-Boot, run either model by either using the VE FVP (default),
+or turning on ``CONFIG_BASE_FVP`` for the more full featured model.
 
-Rather than create a new armv8 board similar to armltd/vexpress64, add
-semihosting calls to the existing one, enabled with CONFIG_SEMIHOSTING
-and CONFIG_BASE_FVP both set. Also reuse the existing board config file
+Rather than create a new armv8 board similar to ``armltd/vexpress64``, add
+semihosting calls to the existing one, enabled with ``CONFIG_SEMIHOSTING``
+and ``CONFIG_BASE_FVP`` both set. Also reuse the existing board config file
 vexpress_aemv8.h but differentiate the two models by the presence or
-absence of CONFIG_BASE_FVP. This change is tested and works on both the
+absence of ``CONFIG_BASE_FVP``. This change is tested and works on both the
 Foundation and Base fastmodel simulators.
 
-The semihosting code adds a command:
+Loading files
+-------------
+
+The semihosting code adds a "hostfs"::
 
   smhload <image> <address> [env var]