RISC-V: introduce ARCH_FOO kconfig aliases for SOC_FOO symbols

To facilitate a transfer from SOC_FOO to ARCH_FOO, over a release cycle,
introduce some aliases so that drivers etc that use the SOC_FOO symbols
can be converted.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
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To me, the most straight-forward conversion looks like so:
- this patch is applied in week 2 of the merge window, to avoid
  any conflicts with the Renesas tree
- all users of the SOC_ variants can be converted over a release cycle
  (or more) & no trees need to merge an immutable branch.
- we convert defconfig etc over after all users are converted
- doing it over at least one release cycle means that `make oldconfig`
  will keep people's configs working as they upgrade
- any new SoC families added uses ARCH_FOO