board: ti: am57xx: Add support for am572x idk in SPL

The AM572x-IDK board (Industrial Dev Kit) is a board based on TI's AM5728x
SOC which has a dual core 1.5GHz A15 processor. This board is a development
platform for the Industrial market with:
- 2GB 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 link to the data sheet and TRM can be found here:

http://www.ti.com/product/AM5728



NOTE: DT support is still pending upstream kernel acceptance but we
should be able to get the base system support with this patch.

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