imx: imx8ulp_evk: Update LPDDR4 PHY settings

Update DDR PHY settings to support LPDDR4 mode only by adjusting
DQ VREF ctrl, ODT and pads drive strength.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>