mb/qemu-riscv: update to match current qemu version

Boots again to payload not found on qemu.

Change-Id: Ie107eb882cbaac5a5a06c1ff990e7b9364377640
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/c/30554
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Documentation/mainboard/emulation/qemu-riscv.md

@@ -0,0 +1,8 @@
+# Qemu RISC-V emulator
+
+## Building coreboot and running it in Qemu
+
+- Configure coreboot and run `make` as usual
+- Run `util/riscv/make-spike-elf.sh build/coreboot.rom build/coreboot.elf` to
+  convert coreboot to an ELF that Qemu can load
+- Run `qemu-system-riscv64 -M virt -m 1024M -nographic -kernel build/coreboot.elf`