ARM: pxa: add .dtsi files

This adds .dtsi files to describe the PXA SoCs. pxa3xx simply augments
pxa2xx. Not all devices are listed yet, and it will need some time to
get all the drivers ported.

For now, pxa27x.dtsi only enables the PXA's interrupt priority feature.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>