riscv: config: enable SOC_STARFIVE in defconfig

SOC_STARFIVE is the odd one out among the (compatible) SOC_FOO options
as it is not enabled in the default defconfig. Add it to make catching
dt regressions etc easier.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Emil Renner Berthing <kernel@esmil.dk>
Link: https://lore.kernel.org/r/20220617182424.324276-1-mail@conchuod.ie


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>