ARM i.MX53: remove platform ahci support

The i.MX53 ahci platform support is unused in mainline. To demotivate
people using it just remove it from the tree.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>