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/include/configs/mx51evk.h b/include/configs/mx51evk.h
index e8c6618..4e82355 100644
--- a/include/configs/mx51evk.h
+++ b/include/configs/mx51evk.h
@@ -235,6 +235,7 @@
 
 #define CONFIG_SYS_DDR_CLKSEL	0
 #define CONFIG_SYS_CLKTL_CBCDR	0x59E35100
+#define CONFIG_SYS_MAIN_PWR_ON
 
 /*-----------------------------------------------------------------------
  * FLASH and environment organization