regulator: pfuze100: add .is_enable() for pfuze100_swb_regulator_ops

If is_enabled() is not defined, regulator core will assume
this regulator is already enabled, then it can NOT be really
enabled after disabled.

Based on Li Jun's patch from the NXP kernel tree.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>