imx28: Fix issue with GCC 5.x

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>