pinctrl: make struct pinfunction a pointer in struct function_desc

pinctrl: make struct pinfunction a pointer in struct function_desc

We currently duplicate the entire struct pinfunction object in
pinmux_generic_add_pinfunction(). While this is inevitable when the
arguments come in split through pinmux_generic_add_function(), users of
pinmux_generic_add_pinfunction() will typically pass addresses of
structures in .rodata, meaning we can try to avoid the duplication with
the help from kmemdup_const(). To that end: don't wrap the entire struct
pinfunction in struct function_desc but rather just store the address.

Tested-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>