arch: riscv: add config option for building SiFive's SoC resource

Create a config option for building SiFive SoC specific resources
e.g. SiFive device tree, platform drivers...

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