regulator: lp872x: Make REGULATOR_LP872X depend on I2C rather than I2C=y

lp872x regulator driver can be built a module after commit 631a1fc
"regulator: build: Allow most regulators to be built as modules".
Make REGULATOR_LP872X depend on I2C to allow building lp872x module when I2C=m.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>