beagle_x15: add board support for Beagle x15

BeagleBoard-X15 is the next generation Open Source
Hardware BeagleBoard based on TI's AM5728 SoC
featuring dual core 1.5GHZ A15 processor. The
platform features 2GB DDR3L (w/dual 32bit busses),
eSATA, 3 USB3.0 ports, integrated HDMI (1920x108@60),
separate LCD port, video In port, 4GB eMMC, uSD,
Analog audio in/out, dual 1G Ethernet.

For more information, refer to:
http://www.elinux.org/Beagleboard:BeagleBoard-X15

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>