omap2/3/4: Introducing 'gpmc-nand.c' for GPMC specific NAND init

Introducing 'gpmc-nand.c' for GPMC specific NAND init.
For example: GPMC timing parameters and all.
This patch also migrates gpmc related calls from 'nand/omap2.c'
to 'gpmc-nand.c'.

Signed-off-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>