ls2080a/dts: Add little endian property for GPIO IP block

The GPIO block for ls2080a platform has little endian registers,
the GPIO driver needs this property to read/write registers by
right interface.

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