arm: pxa: move init functions into generic.h

In order to have a unique .c file for all pxa variants device-tree
definitions, all the initialization functions for MACHINE_START and
DT_MACHINE_START have been put together into generic.h.

The alternative would have been one pxaXXX-dt.c file per variant.

The move is necessary because each include/mach/pxaXXX.h includes the
variant register descriptions which intersects and conflicts one with
each other.

The change is a preparation for pxa-dt.c to support multiple pxa,
ie. pxa3xx and pxa27x.

The machine files including mach/pxaXXX.h all include generic.h, which
guarantees no regression should be introduced.

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