This patch fixes following build warnings.

warning: (MACH_ARMLEX4210) selects SATA_AHCI_PLATFORM
          which has unmet direct dependencies (ATA)

And adds EXYNOX4_DEV_AHCI for building machines which are
not suppoort for AHCI feature on board.
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>