pinctrl: meson: add gen_clk pins

Add the pinctrl configuration for gen_clk on gxbb and axg.
gen_clk allows to output and devide several internal clocks of SoC,
including most of the plls.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>