arch/riscv: Enable FIT support

Tested on qemu-riscv.
Depends on OpenSBI integration and proper memory detection in qemu.

Boots into Linux until initrd should be loaded.

Tested on SiFive/unleashed:
Boots into Linux until earlycon terminates.

Change-Id: I5ebc6cc2cc9e328f36d70fba13555386bb8c29d6
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/30292
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/lib/payloads/fit.md

@@ -6,6 +6,7 @@
 ## Supported architectures
 
 * aarch64
+* riscv
 
 ## Supported FIT sections
 
@@ -24,6 +25,7 @@ The section must be named in order to be found by the FIT parser:
 ## Architecture specifics
 
 The FIT parser needs architecure support.
+
 ### aarch64
 The source code can be found in `src/arch/arm64/fit_payload.c`.
 
@@ -31,6 +33,13 @@ On aarch64 the kernel (a section named 'kernel') must be in **Image**
 format and it needs a devicetree (a section named 'fdt') to boot.
 The kernel will be placed close to "*DRAMSTART*".
 
+### RISC-V
+The source code can be found in `src/arch/riscv/fit_payload.c`.
+
+On RISC-V the kernel (a section named 'kernel') must be in **Image**
+format and it needs a devicetree (a section named 'fdt') to boot.
+The kernel will be placed close to "*DRAMSTART*".
+
 ### Other
 Other architectures aren't supported.