RISC-V: Add StarFive SoC Kconfig option

Add StarFive Kconfig option to select SoC specific and common drivers
required for these SoCs. Select subsystems required to boot so the
required drivers gets enabled by default.

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>