arch/riscv: Pass cbmem_top to ramstage via calling argument

Tested on the Qemu-Virt target both 32 and 64 bit.

Change-Id: I5c74cd5d3ee292931c5bbd2e4075f88381429f72
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36558
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/arch/riscv/index.md

@@ -19,6 +19,9 @@ On entry to a stage or payload (including SELF payloads),
 * all harts are running.
 * A0 is the hart ID.
 * A1 is the pointer to the Flattened Device Tree (FDT).
+* A2 contains the additional program calling argument:
+  - cbmem_top for ramstage
+  - the address of the payload for opensbi
 
 ## Additional payload handoff requirements
 The location of cbmem should be placed in a node in the FDT.