regulator: tps65023: Set missing bit for update core-voltage

Setting the GO bit in CTRL2 for updating the core voltage

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>