[ARM] pxa: Avoid fiddling with CKEN register on suspend

The PXA manuals indicate that when in standby or sleep modes, clocks to
peripherals are shut off by the processor itself.  Eg:

PXA270 standby: "In standby mode, all clocks are disabled except those
 for the power manager and the RTC."

PXA270 sleep: "In sleep mode, all clocks are disabled to the processor
 and to all peripherals except the RTC."

PXA255 sleep: "In Sleep Mode, all processor and peripheral clocks are
 disabled, except the RTC."

Therefore, it should be safe to leave the clock enable register alone
prior to entering low power modes for these SoCs.

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