pinctrl: nomadik: Add new MC2 config

The MC2 MMC interface can be configured with the FBCLK
(feedback clock) unconnected, leaving the pin available
for reuse for something else. Add a group mc2_a_2 for
this.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20220418164317.864418-1-linus.walleij@linaro.org