MFC r274670, r274671, r276168:

MFC r274670, r274671, r276168:

Moves all the duplicate code to a single function.

Verify for invalid modes and unwanted flags before pass the new flags to
driver.

Make gpio_default_map_gpios() static.  No functional changes.

Improves the GPIO API description a little bit.

gpio_pin_max must return the maximum supported pin number and not the total
number of pins on the system.

git-svn-id: svn://svn.freebsd.org/base/stable/10@278786 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f