ARM: omap: fix GPMC address-map size for NAND and NOR devices

Fixes commit a0a37183
    ARM: omap: merge GPMC initialization code for all platform

1) NAND device are not directly memory-mapped to CPU address-space, they are
 indirectly accessed via following GPMC registers:
 - GPMC_NAND_COMMAND_x
 - GPMC_NAND_ADDRESS_x
 - GPMC_NAND_DATA_x
 Therefore from CPU's point of view, NAND address-map can be limited to just
 above register addresses. But GPMC chip-select address-map can be configured
 in granularity of 16MB only.
 So this patch uses GPMC_SIZE_16M for all NAND devices.

2) NOR device are directly memory-mapped to CPU address-space, so its
 address-map size depends on actual addressable region in NOR FLASH device.
 So this patch uses CONFIG_SYS_FLASH_SIZE to derive GPMC chip-select address-map
 size configuration.

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