arm: imx8m: Fix pad DSE issue for i.MX8MM/MN/MP

According to 8MM/MN/MP reference manual, their pad registers only have
4 valid DSE values. And DSE2 and DSE4 are different with current
definitions in iomux-v3.h. Fix the issue to align with manual.

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