pinctrl: max77620: select PINMUX

The recently added max77620 driver fails to build when CONFIG_PINMUX
is not set:

pinctrl/pinctrl-max77620.c:272:21: error: variable 'max77620_pinmux_ops' has initializer but incomplete type
 static const struct pinmux_ops max77620_pinmux_ops = {
                     ^~~~~~~~~~
pinctrl/pinctrl-max77620.c:273:2: error: unknown field 'get_functions_count' specified in initializer

This adds the Kconfig 'select' statement that was clearly meant
to be there and is used in all other pinmux drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>