ata: sata_dwc_460ex: use "dmas" DT property to find dma channel

Currently this driver only works with a DesignWare DMA engine which it
registers manually using the second "reg" address range and interrupt
number from the DT node.

This patch makes the driver instead use the "dmas" property if present,
otherwise optionally falling back on the old way so existing device
trees can continue to work.

With this change, there is no longer any reason to depend on the 460EX
machine type so drop that from Kconfig.

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