spi: spi-imx: only enable the clocks when we start to transfer a message

Current code keeps the clocks enabled all the time, it wastes the power
when there is no operaiton on the spi controller.

In order to save the power, this patch adds the two hooks:
   spi_imx_prepare_message: enable the clocks for this message
   spi_imx_unprepare_message: disable the clocks.

This patch also disables the clocks in the end of the probe.

Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>