ARM: dts: ls1021a: Add gpio support for ls1021a platform

Add gpio nodes for ls1021a platform dts file. The gpio
IP block of the ls1021a can be supported by the code
drivers/gpio/gpio-mpc8xxx.c.

The compatible "fsl,qoriq-gpio" is used by gpio driver:
drivers/gpio/gpio-mpc8xxx.c to implement general gpio
functionalities.

The chip-specific compatible "fsl,ls1021a-gpio" may be
used to fix potential gpio IP block errata or other
chip-specific gpio issues.

Signed-off-by: Liu Gang <Gang.Liu@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>