powerpc: Implement GPIO driver for simple memory-mapped banks

The driver supports very simple GPIO controllers, that is, when a
controller provides just a 'data' register. Such controllers may be
found in various BCSRs (Board's FPGAs used to control board's
switches, LEDs, chip-selects, Ethernet/USB PHY power, etc).

So far we support only 1-byte GPIO banks. Support for other widths may
be implemented when/if needed.

p.s.
To avoid "made up" compatible entries (like compatible = "simple-gpio"),
boards must call simple_gpiochip_init() to pass the compatible string.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>