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: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>