mxs: Staticize SPL functions

The MXS SPL didn't mark local functions "static". Fix it. This also makes the
SPL smaller by roughly 300 bytes.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>