mmc: omap_hsmmc: claim pinctrl at probe time

This allows DT-driven board to set up the pin mux only when the driver
is in use.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Venkatraman S <svenkatr@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>