RISC-V: stop selecting SIFIVE_PLIC at the SoC level

RISC-V: stop selecting SIFIVE_PLIC at the SoC level

The SIFIVE_PLIC driver is used by all current RISC-V SoCs & will be,
where possible, used for future implementations. Rather than having each
driver select the option on a case-by-case basis, do so at the arch
level.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221118104300.85016-4-conor@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>