mx8mm_evk: spl: Staticize functions

Functions spl_dram_init() and power_init_board() are used only in
the scope of this file, so make them static to fix the following sparse
warnings:

board/freescale/imx8mm_evk/spl.c:40:6: warning: no previous prototype
for ‘spl_dram_init’ [-Wmissing-prototypes]

board/freescale/imx8mm_evk/spl.c:85:5: warning: no previous prototype
for ‘power_init_board’ [-Wmissing-prototypes]

Signed-off-by: Alifer Moraes <alifer.wsdm@gmail.com>