imx: cpu: restrict get_boot_device

i.MX8MN has its own get_boot_device, so restrict with i.MX8MQ and
i.MX8MM.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/mach-imx/cpu.c b/arch/arm/mach-imx/cpu.c
index 4db37c7..51c7c05 100644
--- a/arch/arm/mach-imx/cpu.c
+++ b/arch/arm/mach-imx/cpu.c
@@ -377,7 +377,7 @@
 }
 #endif
 
-#if defined(CONFIG_MX7) || defined(CONFIG_IMX8M)
+#if defined(CONFIG_MX7) || defined(CONFIG_IMX8MQ) || defined(CONFIG_IMX8MM)
 enum boot_device get_boot_device(void)
 {
 	struct bootrom_sw_info **p =