add support for the am335x based boards from siemens:

dxr2:
- DDR3 128MiB
- NAND 256MiB
- Ethernet with external Switch SMSC LAN9303
- no PMIC
- internal Watchdog
- DFU support

pxm2:
- DDR2 512 MiB
- NAND 1024 MiB
- PMIC
- PHY atheros ar803x
- USB Host
- internal Watchdog
- DFU support

rut:
- DDR3 256 MiB
- NAND 256 MiB
- PMIC
- PHY natsemi dp83630
- external Watchdog
- DFU support

Signed-off-by: Heiko Schocher
Signed-off-by: Roger Meier
Signed-off-by: Samuel Egli
Cc: Pascal Bach
Cc: Tom Rini