board: ti: am57xx: Add support for the am571x idk

The AM571x Industrial Development Kit (IDK) is a board based on TI's
AM571x SoC which has a single core 1.5GHz Cortex-A15processor. This
board is a development platform for the Industrial Market with:

- 1GB of DDR3L
- Dual 1Gbps Ethernet
- HDMI
- PRU-ICSS
- uSD
- 16GB eMMC
- CAN
- RS-485
- PCIe
- USB3.0
- Video Input Port
- Industrial IO port and expansion connector

The PRU/ICSS will be supported by 3rd party software for EtherCat,
Profibus, and other Industrial protocols.

The link to the data sheet and TRM can be found here:
http://www.ti.com/product/AM5718



Signed-off-by: Steve Kipisz <s-kipisz2@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>