[ARM] pxa: add pxa2xx_mfp_set_lpm() to facilitate low power state change

Some boards want to change low power state of pins on-the-fly, this
function helps to facilitate that operation instead of switching
back-n-forth between two configurations with pxa2xx_mfp_config().

Signed-off-by: Eric Miao <eric.miao@marvell.com>
Tested-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>