arm, am335x: add support for 3 siemens boards

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 <hs@denx.de>
Signed-off-by: Roger Meier <r.meier@siemens.com>
Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Cc: Pascal Bach <pascal.bach@siemens.com>
Cc: Tom Rini <trini@ti.com>