clk: imx6q: Properly handle imx6qp ECSPI clk_sels

clk: imx6q: Properly handle imx6qp ECSPI clk_sels

The ECSPI clock has the ability to select between pll3_60m and
osc on the imx6qp, where it's fixed on other variants.  Fix this
by adding using a helper function to determine SoC variant and
register the clock accordingly.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>