riscv: defconfig: enable SOC_SIFIVE

Enable SOC_SIFIVE so the default upstream config is bootable on the SiFive
Unleashed Board.
And have basic support for future boards based on the same SoC.

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