ARM: SPL: Move gpmc_init() to spl_board_init()

This is an OMAP/related-specific function, move calling it to
spl_board_init() and turn on CONFIG_SPL_BOARD_INIT on the boards that
enabled NAND and didn't enable this already.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/include/configs/mcx.h b/include/configs/mcx.h
index 733022e..a841c70 100644
--- a/include/configs/mcx.h
+++ b/include/configs/mcx.h
@@ -359,6 +359,7 @@
 
 /* Defines for SPL */
 #define CONFIG_SPL
+#define CONFIG_SPL_BOARD_INIT
 #define CONFIG_SPL_NAND_SIMPLE
 #define CONFIG_SPL_NAND_SOFTECC