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
diff --git a/plat/imx/imx8m/imx8mn/include/gpc_reg.h b/plat/imx/imx8m/imx8mn/include/gpc_reg.h
index fd10438..8a81368 100644
--- a/plat/imx/imx8m/imx8mn/include/gpc_reg.h
+++ b/plat/imx/imx8m/imx8mn/include/gpc_reg.h
@@ -106,4 +106,6 @@
 #define GPUMIX_PGC			0xdc0
 #define DISPMIX_PGC			0xe80
 
+#define IRQ_IMR_NUM			U(4)
+
 #endif /* GPC_REG_H */