ARM: dts: am57xx-beagle-x15: Add support for rev C

Latest update to the BeagleBoard-X15 platform (revision C). This board contains
a silicon update (Rev 2.0), which includes a fix for the 2nd ethernet phy when
running at 1000 Mbps speeds.

This board can be indentified by the [C.00] after [BBRDX15_] in the at24 eeprom:
[BBRDX15_C.001731PX150249]

Rev C is now in full production and boards are available for end users.

https://beagleboard.org/x15
https://github.com/beagleboard/beagleboard-x15/



Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
CC: Tony Lindgren <tony@atomide.com>
CC: Nishanth Menon <nm@ti.com>
CC: Lokesh Vutla <lokeshvutla@ti.com>
CC: Kishon Vijay Abraham I <kishon@ti.com>
CC: Jason Kridner <jkridner@beagleboard.org>
CC: Drew Fustini <drew@beagleboard.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>