libata: make AHCI_XGENE depend on PHY_XGENE

AHCI_XGENE is only applicable on ARM64 but it can also be enabled for
compile testing; however, AHCI_XGENE selects PHY_XGENE which has other
arch specific dependencies.  This leads to the following warning when
enabling it on other archs for compile testing.

  warning: (AHCI_XGENE) selects PHY_XGENE which has unmet direct
  dependencies (HAS_IOMEM && OF && (ARM64 || COMPILE_TEST))

Selecting a config option which itself has dependencies can easily
lead to broken configurations.  For now, let's just make AHCI_XGENE
depend on PHY_XGENE which has all the necessary dependencies already.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Loc Ho <lho@apm.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>