MFC r273917, r273926:

MFC r273917, r273926:

Fix the gpiobus locking by using a more sane model where it isn't necessary
hold the gpiobus lock between the gpio calls.

gpiobus_acquire_lock() now accepts a third parameter which tells gpiobus
what to do when the bus is already busy.

When GPIOBUS_WAIT wait is used, the calling thread will be put to sleep
until the bus became free.

With GPIOBUS_DONTWAIT the calling thread will receive EWOULDBLOCK right
away and then it can act upon.

This fixes the gpioiic(4) locking issues that arises when doing multiple
concurrent access on the bus.

Fix the build of non-FDT systems by moving the gpiobusvar.h header outside
the FDT #ifdef.

While here remove a few unused headers.

git-svn-id: svn://svn.freebsd.org/base/stable/10@278783 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f