[ARM] pxa: add ability to set suspend mode

PXA processors have several low-power modes. Currently kernel supports
only one of these modes for PM_SUSPEND_MEM.

This patch adds ability to set desired suspend mode for PXA27x based
machines.

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Reviewed-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Eric Miao <eric.miao@marvell.com>