[ARM] pxa: Allow platforms to override PSPR setting

Currently, we set PSPR just before entering sleep mode.  However,
some platforms have different requirements for setting PSPR in
order to properly wake up.

Set PSPR earlier in the suspend cycle so that platforms can
change the setting by using a sysdev driver instead.

Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>