mfd: Allow WM8994 LDO enable pulls to be disabled

In systems where the LDO enables are always driven (for example, being
connected to an always on supply rail or a GPIO which is driven by the
CPU even in suspend) then we can disable the pull downs on the LDO for
a small power savings.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>