ARM: 8569/1: pl2x0: Add OF control of cache power management

Add ability to override power management bits of 310 controllers
(dynamic clock gating and standby mode) through OF entries. As the
saved register is only applied when working on a supported controller,
it is safe to save the settings.

In order to maintain existing behavior, if the settings are not found
in the DT, the corresponding feature will be enabled.

Signed-off-by: Brad Mouring <brad.mouring@ni.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>