pinctrl: bcm: add OF dependencies

Building without CONFIG_OF gives us these warnings for the broadcom
pinctrl drivers:

drivers/pinctrl/bcm/pinctrl-nsp-mux.c:356:20: error: 'pinconf_generic_dt_node_to_map_group' undeclared here (not in a function)
drivers/pinctrl/bcm/pinctrl-cygnus-mux.c:739:20: error: 'pinconf_generic_dt_node_to_map_group' undeclared here (not in a function)

The function is only available when CONFIG_OF is set, so we should add
a Kconfig dependency for both drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: cc4fa83f ("pinctrl: nsp: add pinmux driver support for Broadcom NSP SoC")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>