ata: Add iMX pata support

Add basic support for pata on iMX. It has been tested only on imx51.
SDMA support will probably be added later so this version supports only
PIO.

v2:
  - enable only when needed IORDY
  - use dev_get_drvdata
v3:
  - add missing clk_put() calls
  - use platform_get_irq()
  - fix resume code to avoid disabling IORDY on resume
v4:
  - Remove EXPERIMENTAL and switch to depends on ARCH_MXC
  - Use devm_kzalloc()
  - make clock a must-have
  - Use only 1 ioremap

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>