The Kindle Model No. D01100 (Kindle Wi-Fi), D01200 (Kindle Touch)
and EY21 (Kindle Paperwhite) are refered as the Kindle 4th and 5th generation.
Those models are based on an i.MX50 SoC and use LPDDR1 or LPDDR2 Memory.

The devices boot in internal boot mode from a build-in eMMC, alternatively
some devices may be set into USB-downloader mode by pressing a specific key
at startup.

Add support for the i.MX50 based Kindle device and make barebox a drop-in
replacement for the factory shipped u-boot image.

Notable features:
- Support for eMMC, USB, UART, I2C, SPI and Keys (except keyboard).
- LPDDR1 and LPDDR2 setup is done via DCD, the same imximage may be used
  for USB-startup and for installation.
- Support for vendor specific ATAGs that are required for the Kindle-System.
- usbserial barebox console access by pressing Select button at startup,
  alternatively full console support on connector J14.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>