mxs: Use correct function name to initialize dram

mxs: Use correct function name to initialize dram

commit d92591a (mxs: Convert sys_proto.h prefixes to 'mxs') introduced
a mxs_dram_init() function, which is not used anywhere.

Fix it, so that the following warning goes away:

mx28evk.c: In function ‘dram_init’:
mx28evk.c:67:2: warning: implicit declaration of function ‘mx28_dram_init’ [-Wimplicit-function-declaration]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>