ARM: pxa: remove unused mach/bitfield.h

The sa1111.h header defines some constants using the bitfield
macros, but those are only used on sa1100, not on pxa, and the
users include the bitfield header through mach/hardware.h.

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