[ARM] pxa: add support for the IskraTel XCEP board

Added support for the Iskratel XCEP single board computer, embedded in
instruments used in particle accelerators.

The patch contains the platform specific code, Makefile and Kconfig
changes for platform arm-pxa.

Signed-off-by: Aleš Bardorfer <ales.bardorfer@i-tech.si>
Signed-off-by: Michael Abbott <michael.abbott@diamond.ac.uk>
Signed-off-by: Matej Kenda <matej.kenda@i-tech.si>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>