powerpc: Add device tree bindings for BCSR GPIO banks

The patch adds bindings for BCSR GPIO banks, the bindings are used to
describe particular BCSR registers that act as simple GPIO controllers.
These GPIO banks might control power switches, SPI chip-selects, LEDs,
etc.

While at it, also fix "length" spelling error in the PIXIS FPGA
bindings.

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