riscv: Add Microchip MPFS Icicle board support

This patch adds Microchip MPFS Icicle board support.
For now, NS16550 serial driver is only enabled.
The Microchip MPFS Icicle defconfig by default builds
U-Boot for M-Mode with SMP support.

Signed-off-by: Padmarao Begari <padmarao.begari@microchip.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>