spi: imx: Use clock framework if enabled

In case the clock framework is enabled, enable the SPI controller clock
and obtain max frequency from the clock framework.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jagan Teki <jagan@amarulasolutions.com>
Cc: Stefano Babic <sbabic@denx.de>