power: twl4030: Add CONFIG_CMD_POWEROFF support

With the addition of twl4030_power_off(), let's allow the 'poweroff' command
to run this function when CONFIG_CMD_POWEROFF is enabled.

Tested on a DM3730 with twl4030 PMIC.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>