pinctrl: ralink: move ralink architecture pinmux header into the driver

Ralink architecture is making use of the header located in
'arch/mips/include/asm/mach-ralink/pinmux.h' to stablish the
mechanisms to make derived SoCs to set its pin functions and
groups. In order to move all architecture pinmux into a more
accurate place which is 'drivers/pinctrl/ralink' we have to
first of all move this file also there with a small modification
which creates 'rt2880_pinmux_init' function to allow SoCs pinctrl
drivers to pass its configuration to the common code located in
'pinctrl-rt2880.c' file.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210604115159.8834-2-sergio.paracuellos@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>