arm: imx6: Switch DDR3 calibration to wait_for_bit()

Switch the DDR3 calibration from ad-hoc implementation of wait_for_bit()
to generic implementation of wait_for_bit().

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>