arm: pxa: update LP-8x4x to boot DT kernel

DT kernel requires CONFIG_OF_LIBFDT. 'bootm' needs to know DT location.
In addition, fix kernel console device and enable U-Boot long help.

Signed-off-by: Sergei Ianovich <ynvich@gmail.com>
CC: Marek Vasut <marex@denx.de>