arm: pxa: remove unnecessary includes from pxa-dt

As the init functions necessary for machine init have moved to
generic.h, remove the unnecessary includes and prototypes definitions
from pxa-dt.c.

This removes the include of mach/pxaXXX-regs.h, and make pxa-dt generic
enough to accept other pxa variants.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Arnd Bergmann <arnd@arndb.de>