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/gpc_common.c b/plat/imx/imx8m/gpc_common.c
index 8aae1a6..babcecf 100644
--- a/plat/imx/imx8m/gpc_common.c
+++ b/plat/imx/imx8m/gpc_common.c
@@ -16,7 +16,7 @@
 #include <imx8m_psci.h>
 #include <plat_imx8.h>
 
-static uint32_t gpc_imr_offset[] = { 0x30, 0x40, 0x1c0, 0x1d0, };
+static uint32_t gpc_imr_offset[] = { IMR1_CORE0_A53, IMR1_CORE1_A53, IMR1_CORE2_A53, IMR1_CORE3_A53, };
 
 #pragma weak imx_set_cpu_pwr_off
 #pragma weak imx_set_cpu_pwr_on