mtd: nand: omap: merge duplicate GPMC data from different arch-xx headers into common omap_gpmc.h

Each SoC platform (AM33xx, OMAP3, OMAP4, OMAP5) has its own copy of GPMC related
defines and declarations scattered in SoC platform specific header files
like include/asm/arch-xx/cpu.h
However, GPMC hardware remains same across all platforms thus this patch merges
GPMC data scattered across different arch-xx specific header files into single
header file include/asm/arch/omap_gpmc.h

Build tested using: ./MAKEALL -s am33xx -s omap3 -s omap4 -s omap5

Signed-off-by: Pekon Gupta <pekon@ti.com>