arm: Add extra boot device (UART) to run Ymodem u-boot.img boot on XEA (imx28)

arm: Add extra boot device (UART) to run Ymodem u-boot.img boot on XEA (imx28)

This commit enables imx28 based XEA board's u-boot.sb (SPL) to download
u-boot proper (u-boot.img) via Ymodem protocol.

This is extremely useful in the recovery scenario where u-boot.sb is
downloaded via uuu utility to SDRAM [*], and then one can upload u-boot
proper via serial console to fully debrick the device.

Note - debricking procedure of imx28 devices:
- NXP's original USB based tools (like mxsldr or uuu) expect single
  u-boot.sb which is a relic of the old U-Boot (~2013) without SPL and
  U-Boot proper distinction.

[*] On Host:
------------
cat << EOF > imx28_xea.lst
uuu_version 1.3.0
SDPS: boot -f /srv/tftp/xea/u-boot.sb
SDPU: done
EOF

Please start picocom:
sudo picocom -b 115200 -s "sz -vv" /dev/ttyUSB1
sudo ./uuu/uuu -V imx28_xea.lst

On the U-boot console one shall see:
Trying to boot from UART
CCC

Then please press CTRL+A, S
and type u-boot.img

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Peng Fan <peng.fan@nxp.com>