ahci: imx: Pull out the clock enable/disable calls

The same code for enabling and disabling SATA clock was found in multiple
places in the driver. Implement functions that enable/disable the SATA clock
and use them in such places instead of duplicating the code.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Shawn Guo <shawn.guo@linaro.org>
Cc: Richard Zhu <r65037@freescale.com>
Cc: Linux-IDE <linux-ide@vger.kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>