riscv: select SiFive platform drivers with SOC_SIFIVE

On selection of SOC_SIFIVE select the corresponding platform drivers.

Signed-off-by: Loys Ollivier <lollivier@baylibre.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>