RISC-V: drop SOC_VIRT for ARCH_VIRT

RISC-V: drop SOC_VIRT for ARCH_VIRT

The ARCH_ and SOC_ versions of this symbol have persisted for quite a
while now in parallel. Generated .config files from previous LTS kernels
should have both. Finally remove SOC_VIRT and update all config files
using it.

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>