ARM: mxs: make lowlevel_init() weak

With the full SPL framework enabled, lowlevel_init() is required.
Make the empty stub weak so boards can override it.

Signed-off-by: Mans Rullgard <mans@mansr.com>

arch/arm/cpu/arm926ejs/mxs/mxs.c

@@ -24,7 +24,7 @@
 DECLARE_GLOBAL_DATA_PTR;
 
 /* Lowlevel init isn't used on i.MX28, so just have a dummy here */
-void lowlevel_init(void) {}
+__weak void lowlevel_init(void) {}
 
 void reset_cpu(ulong ignored) __attribute__((noreturn));