ARM: omap: merge GPMC initialization code for all platform

GPMC controller on TI's OMAP SoC is general purpose controller to interface
with different types of external devices like;
 - parallel NOR flash
 - parallel NAND flash
 - OneNand flash
 - SDR RAM
 - Ethernet Devices like LAN9220

Though GPMC configurations may be different for each platform depending on
clock-frequency and external device interfacing with controller. But
initialization sequence remains common across all platfoms.

Thus this patch merges gpmc_init() scattered in different arch-xx/mem.c
files into single omap-common/mem-common.c

However, actual platforms specific register config values are still sourced
from corresponding platform specific headers like;
 AM33xx: arch/arm/include/asm/arch-am33xx/mem.h
 OMAP3:  arch/arm/include/asm/arch-omap3/mem.h
 OMAP4:  arch/arm/include/asm/arch-omap4/mem.h
 OMAP4:  arch/arm/include/asm/arch-omap5/mem.h

Also, CONFIG_xx passed by board-profile decide config for which set of macros
need to be used for initialization
 CONFIG_NAND:    initialize GPMC for NAND device
 CONFIG_NOR:     initialize GPMC for NOR device
 CONFIG_ONENAND: initialize GPMC for ONENAND device

Signed-off-by: Pekon Gupta <pekon@ti.com>
[trini: define GPMC_SIZE_256M for omap3]
Signed-off-by: Tom Rini <trini@ti.com>