pinctrl: Modify Kconfig to fix linker error

Fix below linker error

    ld: drivers/pinctrl/pinctrl-equilibrium.o: in function
    `pinconf_generic_dt_node_to_map_all':
    pinctrl-equilibrium.c:(.text+0xb): undefined reference
    to `pinconf_generic_dt_node_to_map'

Caused by below commit

    1948d5c5 ("pinctrl: Add pinmux & GPIO controller driver for a new SoC")

by adding 'depends on OF' in Kconfig driver entry.

Reported-by: Randy Dunlap <rdunlap@infradead.org&gt;>
Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Link: https://lore.kernel.org/r/ba937f271d1a2173828a2325990d62cb36d61595.1575514110.git.rahul.tanwar@linux.intel.com


Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>