ARM: pxa: restore mfp-pxa320.h

This file was removed in an earlier commit but is actually still
needed, so restore it.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Fixes: d6df7df7

 ("ARM: pxa: remove unused board files")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>