regulator: lp872x: set the default DVS mode

 The lp872x driver has the DVS platform data which select the register address
 of the BUCK voltage.
 This patch enables updating the default DVS mode when the DVS platform data
 is not defined.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>