ARM: pxa: add dummy clock for pxa25x and pxa27x

gpio-pxa driver is shared among arch-pxa and arch-mmp. Clock is the
essential component on pxa3xx/pxa95x and arch-mmp. So we need to
define dummy clock in pxa25x/pxa27x instead.

This regression was introduced by the commit "ARM: pxa: add dummy
clock for sa1100-rtc", id a55b5adaf403c4d032e0871ad4ee3367782f4db6.

Reported-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Paul Parsons <lost.distance@yahoo.com>
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>