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/imx8mp/include/gpc_reg.h b/plat/imx/imx8m/imx8mp/include/gpc_reg.h
index 12da6ac..7909937 100644
--- a/plat/imx/imx8m/imx8mp/include/gpc_reg.h
+++ b/plat/imx/imx8m/imx8mp/include/gpc_reg.h
@@ -146,4 +146,6 @@
 #define MEDIAMIX_ISPDWP_PGC		0xf80
 #define DDRMIX_PGC			0xfc0
 
+#define IRQ_IMR_NUM			U(5)
+
 #endif /* GPC_REG_H */