cmd: mmc: allow to write protect single boot partition

add arguments for mmc wp to assign which boot partition to protect.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>