riscv: Add Microchip MPFS Icicle Kit support

This patch adds Microchip MPFS Icicle Kit support. For now, only
NS16550 Serial, Microchip clock, Cadence eMMC and MACB drivers are
enabled. The Microchip MPFS Icicle defconfig by default builds
U-Boot for S-Mode because U-Boot on Microchip PolarFire SoC will run
in S-Mode as payload of HSS + OpenSBI.

Signed-off-by: Padmarao Begari <padmarao.begari@microchip.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>