pinctrl: add generic functions + pins mapper

pinctrl: add generic functions + pins mapper

Add a generic function to allow creation of groups and functions at
runtime based on devicetree content, before setting up mux mappings.
It works similarly to pinconf_generic_dt_node_to_map(), and
therefore parses pinconf properties and maps those too, allowing it
to be used as the dt_node_to_map member of the pinctrl_ops struct.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>