[ARM] pxa: add MFP_PULL_FLOAT

There is currently an uncovered case for MFP configuration on PXAs which
is selected by setting the PULL_SEL bit but none of the PULL{UP,DOWN}_EN
bits. This case is needed to explicitly let pins float, even if the
selected alternate function would default to a configuration with a pull
resistor enabled.

Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>