pinctrl: add bcm2835 driver

The BCM2835 GPIO module is a combined GPIO controller, (GPIO) interrupt
controller, and pinmux/control device.

Original driver by Simon Arlott.
Rewrite including GPIO chip device by Chris Boot.

Upstreaming changes by Stephen Warren:
* Wrote DT binding documentation.
* Changed brcm,function to an integer to more directly match the
  datasheet, and to match brcm,pins being an integer.
* Implemented pull-up/down pin config.
* Removed read-only DT property and related code. The restriction this
  implemented are driven by the board, not the GPIO HW block, so don't
  really make sense of a HW block binding, were in general incomplete
  (since they could only know about the few pins hard-coded into the
  Raspberry Pi B board design and not the uncommitted GPIOS), and are
  better represented simply by not writing incorrect data into pin
  configuration nodes.
* Don't set GPIO_IN function select in gpio_request_enable() to avoid
  glitches; defer this to gpio_set_direction(). Consequently, removed
  empty bcm2835_pmx_gpio_request_enable().
* Simplified enabled_irq_map[]; make it explicitly 1 entry per bank.
* Lifted use of enabled_irq_map[] outside the per-interrupt loop in
  IRQ handler, thus fixing an issue where the code was indexing into
  enabled_irq_map[] by intra-bank GPIO ID, not global GPIO ID.
* Removed locking in IRQ handler, since all other code uses
  spin_lock_irqsave() and so guarantees it doesn't run concurrently
  with the handler.
* Moved duplicated BUILD_BUG_ON()s into probe(). Also check size of
  bcm2835_gpio_pins[].
* Remove range-checking from bcm2835_pctl_get_groups_count() since we've
  decided to trust the pinctrl core.
* Made bcm2835_pmx_gpio_disable_free() call bcm2835_pinctrl_fsel_set()
  directly for simplicity.
* Fixed body of dt_free_map() to match latest dt_node_to_map().
* Removed GPIO ownership check from bcm2835_pmx_enable() since the pinctrl
  core owns doing this.
* Made irq_chip and pinctrl_gpio_range .name == MODULE_NAME so it's more
  descriptive.
* Simplified remove(); removed call to non-existent
  pinctrl_remove_gpio_range(), remove early return on error.
* Don't force gpiochip's base to 0. Set gpio_range.base to gpiochip's
  base GPIO number.
* Error-handling cleanups in probe().
* Switched to module_platform_driver() rather than open-coding.
* Made pin, group, and function names lower-case.
* s/broadcom/brcm/ in DT property names.
* s/2708/2835/.
* Fixed a couple minor checkpatch warnings, and other minor cleanup.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: Chris Boot <bootc@bootc.net>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>