MLK-12576 imx: imx6ul: disable POR_B internal pull up

From TO1.1, SNVS adds internal pull up control for POR_B,
the register filed is GPBIT[1:0], after system boot up,
it can be set to 2b'01 to disable internal pull up.
It can save about 30uA power in SNVS mode.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>