RISC-V: kbuild: convert all use of SOC_FOO to ARCH_FOO

Convert all non user visible use of SOC_FOO symbols to their ARCH_FOO
variants. The canaan DTs are an outlier in that they're gated at the
directory and the file level. Drop the directory level gating while we
are swapping the symbol names over.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>