spi: omap2-mcspi: add pinctrl support

Adds pinctrl support to support OMAP platforms that boot from DT
and rely on pinctrl support to set pinmuxes.

Signed-off-by: Matt Porter <mporter@ti.com>
Acked-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>