[ARM] pxa: move pm_ops structure into CPU specific files

Move the pm_ops structure into the PXA25x and PXA27x support
files.  Remove the old pxa_pm_prepare() function, and rename
the both pxa_cpu_pm_prepare() functions as pxa_pm_prepare().
We'll fix that later.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>