mxs: Rename 'mx28_dram_init' to 'mxs_dram_init'

The DRAM initialization, after SPL has complete, is exactly the same
for all mxs SoCs so we should name it accordinly.

The following boards has been changed:

 * apx4devkit
 * m28evk
 * mx28evk
 * sc_sps_1

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Acked-by: Marek Vasut <marex@denx.de>
Acked-by: Veli-Pekka Peltola <veli-pekka.peltola@bluegiga.com>
diff --git a/board/denx/m28evk/m28evk.c b/board/denx/m28evk/m28evk.c
index 74da3ea..9473d10 100644
--- a/board/denx/m28evk/m28evk.c
+++ b/board/denx/m28evk/m28evk.c
@@ -72,7 +72,7 @@
 
 int dram_init(void)
 {
-	return mx28_dram_init();
+	return mxs_dram_init();
 }
 
 #ifdef	CONFIG_CMD_MMC