dt-bindings: define little-endian property for QorIQ GPIO

The GPIO block on different QorIQ chips could have registers in different
endianess.  Define the property to specify which endian is used by the
hardware.

Signed-off-by: Liu Gang <Gang.Liu@freescale.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Li Yang <leoli@freescale.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>