imx28: Fix issue with GCC 5.x

The semantics for non-static functions declared inline have changed in
gcc5, causing the empty functions not to be emitted as an external
symbol.

Since lowlevel_init() is only referenced from start.S, it should not be
declared inline at all.

Reported-by: Otavio Salvador <otavio@ossystems.com.br>
Tested-by: Otavio Salvador <otavio@ossystems.com.br>
[trini: Reword commit message]
Signed-off-by: Tom Rini <trini@konsulko.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 */
-inline void lowlevel_init(void) {}
+void lowlevel_init(void) {}
 
 void reset_cpu(ulong ignored) __attribute__((noreturn));