ata: sata_dwc_460ex: add __iomem to register base pointer

The pointer to the mmio register base is missing the __iomem
annotation.  Fix this.

Tested-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Tejun Heo <tj@kernel.org>