ahci_imx: add ahci sata support on imx platforms

imx6q contains one Synopsys AHCI SATA controller, But it can't share
ahci_platform driver with other controllers because there are some
misalignments of the generic AHCI controller - the bits definitions of
the HBA registers, the Vendor Specific registers, the AHCI PHY clock
and the AHCI signals adjustment window(GPR13 register).

 - CAP_SSS(bit20) of the HOST_CAP is writable, default value is '0',
   should be configured to be '1'

 - bit0 (only one AHCI SATA port on imx6q) of the HOST_PORTS_IMPL
   should be set to be '1'.(default 0)

 - One Vendor Specific register HOST_TIMER1MS(offset:0xe0) should be
   configured regarding to the frequency of AHB bus clock.

 - Configurations of the AHCI PHY clock, and the signal parameters of
   the GPR13

Setup its own ahci sata driver, contained the imx6q specific
initialized codes, re-use the generic ahci_platform driver, and keep
the generic ahci_platform driver clean as much as possible.

tj: patch description reformatted

Signed-off-by: Richard Zhu <r65037@freescale.com>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Tejun Heo <tj@kernel.org>