ARM: dts: Add initial LS1021A IoT board dts support

The LS1021A-IoT gateway reference design is a purpose-built,
small footprint hardware platform equipped with a wide array
of both high-speed connectivity and low speed serial interfaces.

CC: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Changming Huang <jerry.huang@nxp.com>
Signed-off-by: Alison Wang <alison.wang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>