ARM: dts: Add am57xx-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 (1920x1080@60), separate LCD
port, video In port, 4GB eMMC, uSD, Analog audio in/out, dual 1G
Ethernet.

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

AM5728 is part of the Sitara product family whose additional details
will be available: http://www.ti.com/lsds/ti/arm/overview.page

Technical Reference Manual for AM5728 is public domain at:
http://www.ti.com/lit/spruhz6



Just add basic support for the moment, the following updates are needed:
  i)	Ethernet - depends on SoC dts fixes
  ii)	USB Client (USB2) - depends on GPIO extcon
  ii)	HDMI - additional driver fixes pending
  iii)	Audio - additional driver fixes pending

NOTE:
AM5728 Data Manual (SPRS915L - August 2014) section 4.1.1 states: "All
unused power supply balls must be supplied with the voltages specified
in the Section 5.2, Recommended Operating Conditions". This implies
that all unused voltage rails for AM5728 can never be switched off even
if the hardware blocks inside that voltage domain is unused. Switching
off these unused rails may result in stability issues on other domains
and increased leakage and power-on-hour impacts.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Tom Rini <trini@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>