omap3 nand: fix issue in board file to detect nand

Board file modified for not to provide gpmc phys_base address to nand driver.
The gpmc_nand_init funciton is now used to detect the nand and required to
adopt _prob function as in nand/omap2.c

Signed-off-by: Sukumar Ghorai <s-ghorai@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>