ARM: PXA2xx: MFP: fix bug with MFP_LPM_KEEP_OUTPUT

Pins that have MFP_LPM_KEEP_OUTPUT set and are configured for output
must retain the output state in low power mode.
Currently, the pin direction configuration is overrided with values
in gpdr_lpm[] array and do not obey the MFP_LPM_KEEP_OUTPUT setting.

Fix the above bug and add some documentation to clarify the
MFP_LPM_KEEP_OUTPUT setting purpose.

Reported-by: Paul Parsons <lost.distance@yahoo.com>
Signed-off-by: Igor Grinberg <grinberg@compulab.co.il>
Tested-by: Paul Parsons <lost.distance@yahoo.com>
Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>