specs: add riscv32 and riscv64 to the list

The spelling of the names is taken from [1].
It also matches "RISC-V 32-bit" and "RISC-V 64-bit" used in
specs/discoverable_partitions_specification.md.

[1] https://github.com/systemd/systemd/blob/19e980eb92d923a25f0cbc38b945c282bc5f5b1b/src/basic/architecture.c#L93

specs/extension_image.md

@@ -125,6 +125,8 @@ Valid values:
 |ppc-le|
 |ppc64|
 |ppc64-le|
+|riscv32|
+|riscv64|
 |s390|
 |s390x|
 |sh|