ARM: OMAP2+: nand: Make board_onenand_init() visible to board code

board_onenand_init() and board_nand_init() initialization functions are
used to initialize OneNAND and NAND memories respectively. But only
board_nand_init() was visible to be used from board code. This patch makes
possible to initialize a OneNAND flash memory within platform code.

Signed-off-by: Javier Martinez Canillas <javier@dowhile0.org>
Acked-by: Enric Balletbo i Serra <eballetbo@gmail.com>
Tested-by: Enric Balletbo i Serra <eballetbo@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>