doc: document sbi command

Add a man-page for the sbi command.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

MAINTAINERS

@@ -951,6 +951,7 @@ S:	Maintained
 T:	git https://gitlab.denx.de/u-boot/custodians/u-boot-riscv.git
 F:	arch/riscv/
 F:	cmd/riscv/
+F:	doc/usage/sbi.rst
 F:	drivers/timer/andes_plmt_timer.c
 F:	drivers/timer/sifive_clint_timer.c
 F:	tools/prelink-riscv.c

doc/usage/index.rst

@@ -19,3 +19,4 @@ Shell commands
    button
    mbr
    pstore
+   sbi

doc/usage/sbi.rst

+.. SPDX-License-Identifier: GPL-2.0+
+
+sbi command
+===========
+
+Synopsis
+--------
+
+::
+
+    sbi
+
+Description
+-----------
+
+The sbi command is used to display information about the SBI (Supervisor Binary
+Interface) implementation on RISC-V systems.
+
+The output may look like:
+
+::
+
+    => sbi
+    SBI 0.2
+    OpenSBI
+    Extensions:
+      sbi_set_timer
+      sbi_console_putchar
+      sbi_console_getchar
+      sbi_clear_ipi
+      sbi_send_ipi
+      sbi_remote_fence_i
+      sbi_remote_sfence_vma
+      sbi_remote_sfence_vma_asid
+      sbi_shutdown
+      SBI Base Functionality
+      Timer Extension
+      IPI Extension
+      RFENCE Extension
+      Hart State Management Extension
+
+The first line indicates the version of the RISC-V SBI specification.
+The second line indicates the implementation.
+The further lines enumerate the implemented extensions.
+
+Configuration
+-------------
+
+To use the sbi command you must specify CONFIG_CMD_SBI=y.