doc: board: Add Microchip MPFS Icicle Kit doc

This doc describes the procedure to build, flash and
boot Linux using U-boot on Microchip MPFS Icicle Kit.

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>