plat: imx8m: Correct the imr mask reg offset

The number of gpc imr mask reg & the offset is different
on some SOC, so correct it & replace the magic number with
macro define.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Change-Id: Ic701675cdd92e043dcd7f06722f2e871068aec74
6 files changed