mxs: Fix VDDx brownout interrupt disable/enable

Incorrect registers HW_POWER_VDDIOCTRL, HW_POWER_VDDACTRL
and HW_POWER_VDDDCTRL are used in the current code to disable/enable
brownout interrupts in 'mxs_power_set_vddx()'.
Change register to HW_POWER_CTRL which contains brownout interrupt
enable bits ENIRQ_VDDIO_BO, ENIRQ_VDDA_BO and ENIRQ_VDDD_BO.

Signed-off-by: Cody Green <cody@londelec.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Lukasz Majewski <lukma@denx.de>