doc: board: Update Microchip MPFS Icicle Kit doc

UART1 use for U-Boot and Linux console instead of UART0 and
UART0 is reserved for Hart Software Services(HSS).

Signed-off-by: Padmarao Begari <padmarao.begari@microchip.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>