Revert "riscv: Remove CONFIG_PHYS_RAM_BASE_FIXED"

This reverts commit 9b79878c.

The removal of this config exposes CONFIG_PHYS_RAM_BASE for all kernel
types: this value being implementation-specific, this breaks the
genericity of the RISC-V kernel so revert it.
Signed-off-by: NAlexandre Ghiti <alex@ghiti.fr>
Tested-by: NEmil Renner Berthing <kernel@esmil.dk>
Reviewed-by: NJisheng Zhang <jszhang@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: NPalmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/Kconfig

@@ -495,8 +495,13 @@ config STACKPROTECTOR_PER_TASK
 	depends on !GCC_PLUGIN_RANDSTRUCT
 	depends on STACKPROTECTOR && CC_HAVE_STACKPROTECTOR_TLS
 
+config PHYS_RAM_BASE_FIXED
+	bool "Explicitly specified physical RAM address"
+	default n
+
 config PHYS_RAM_BASE
 	hex "Platform Physical RAM address"
+	depends on PHYS_RAM_BASE_FIXED
 	default "0x80000000"
 	help
 	  This is the physical address of RAM in the system. It has to be
@@ -509,6 +514,7 @@ config XIP_KERNEL
 	# This prevents XIP from being enabled by all{yes,mod}config, which
 	# fail to build since XIP doesn't support large kernels.
 	depends on !COMPILE_TEST
+	select PHYS_RAM_BASE_FIXED
 	help
 	  Execute-In-Place allows the kernel to run from non-volatile storage
 	  directly addressable by the CPU, such as NOR flash. This saves RAM