plat: imx8mn: fix the m7 enable check on imx8mn

on i.MX8MN, use the 'CPU_WAIT' to check if the
M7 core is enabled or not, as M7 is always enabled
in SRC.
Signed-off-by: Jacky Bai <ping.bai@nxp.com>

plat/imx/imx8mn/gpc.c

@@ -174,8 +174,7 @@ static unsigned int pu_domain_status;
 
 bool imx_is_m4_enabled(void)
 {
-	return !( mmio_read_32(IMX_SRC_BASE + M4RCR)
-		& SRC_SCR_M4C_NON_SCLR_RST_MASK);
+	return !( mmio_read_32(IMX_IOMUX_GPR_BASE + 0x58) & 0x1);
 }
 
 #define M4_LPA_ACTIVE	0x5555