fdt_pinctrl: Add new methods for gpios

Most of the gpio controller cannot configure or get the configuration
of the pin muxing as it's usually handled in the pinctrl driver.
But they can know what is the pinmuxing driver either because they are
child of it or via the gpio-range property.
Add some new methods to fdt_pinctrl that a pin controller can implement.
Some methods are :
fdt_pinctrl_is_gpio: Use to know if the pin in the gpio mode
fdt_pinctrl_set_flags: Set the flags of the pin (pullup/pulldown etc ...)
fdt_pinctrl_get_flags: Get the flags of the pin (pullup/pulldown etc ...)

The defaults method returns EOPNOTSUPP.

Reviewed by: ian, bcr (manpages)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D23093