board/ti/am335x: add support for BeagleBone Green

board/ti/am335x: add support for BeagleBone Green

SeeedStudio BeagleBone Green (BBG) is clone of the BeagleBone Black (BBB) minus
the HDMI port and addition of two Grove connectors (i2c2 and usart2).

This board can be identified by the 1A value after A335BNLT (BBB) in the at24 eeprom:
1A: [aa 55 33 ee 41 33 33 35  42 4e 4c 54 1a 00 00 00 |.U3.A335BNLT....|]

http://beagleboard.org/green
http://www.seeedstudio.com/wiki/Beaglebone_green

In Mainline Kernel as of:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=79a4e64c679d8a0b1037da174e4aea578c80c4e6

Patch tested on BeagleBone Black (rev C) and BeagleBone Green (production model)

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
CC: Tom Rini <trini@konsulko.com>
CC: Jason Kridner <jkridner@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>