This allows users of the pinctrl driver to specify either

  pinctrl-single,bias-pullup = MPF_PULL_UP;

or

  pinctrl-single,bias-pulldown = MPF_PULL_DOWN;

To activate the pull bits in the MFP registers.
Signed-off-by: Daniel Mack <daniel@zonque.org>
Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>