mx5: Mark lowlevel_init board-specific code

The mx5 lowlevel_init.S contains board-specific code based on the reference
design. Let's keep it since it avoids creating new lowlevel_init files and it
may be used by many boards. But add a config to make it optional in order not to
cause issues on boards not following this part of the reference design.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Matt Sealey <matt@genesi-usa.com>
Acked-by: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/cpu/armv7/mx5/lowlevel_init.S b/arch/arm/cpu/armv7/mx5/lowlevel_init.S
index 29ec957..6d9396a9 100644
--- a/arch/arm/cpu/armv7/mx5/lowlevel_init.S
+++ b/arch/arm/cpu/armv7/mx5/lowlevel_init.S
@@ -396,7 +396,7 @@
 	mov r10, lr
 	mov r4, #0	/* Fix R4 to 0 */
 
-#if defined(CONFIG_MX51)
+#if defined(CONFIG_SYS_MAIN_PWR_ON)
 	ldr r0, =GPIO1_BASE_ADDR
 	ldr r1, [r0, #0x0]
 	orr r1, r1, #1 << 23