cmd: Add Kconfig option for multiprocessor cmds

Add Kconfig option(CONFIG_CMD_MP) to enable or disable multiprocessor
commands. Compile cmd/mp.c based on CONFIG_CMD_MP.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>