RISC-V: Add Microchip PolarFire SoC kconfig option

Add Microchip PolarFire kconfig option which selects SoC specific
and common drivers that is required for this SoC.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>