pinctrl: add new generic groups/function creation function for pinmux

pinctrl: add new generic groups/function creation function for pinmux

Akin to my recently added pinctrl_generic_pins_functions_dt_node_to_map(),
create an analogue that performs the same role of dynamically creating
groups at runtime for controllers using the pinmux property.
The pinmux property is freeform, so this function mandates that the
upper 16 bits contain the pin and the lower 16 bits contains the mux
setting. The group's data pointer is populated with an array of the mux
settings for each pin it contains.

Since the node parsing and subsequent pinctrl core function calls are
practically identical to the pins + functions case, other than which
properties are examined, it makes sense to extract the common code from
pinctrl_generic_pins_function_dt_node_to_map() into a generic function
that takes the case-specific devicetree parsing function as an argument.

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