ata: ahci-imx: Covert to use GPIO descriptor

This converts the i.MX AHCI driver to use a GPIO descriptor
instead of parsing the device tree by itself.

This driver is quite obviously device tree only, and the
GPIO line is treated as optional, so let's keep it as
optional.

None of the device trees in the kernel use this GPIO
facility today, so it is hard to test.

Cc: Egor Starkov <egor.starkov@ge.com>
Cc: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>