dt-bindings: regulator: pca9420: reorganize headers/macros

pca9420_i2c.h has neem remamed to pca9420.h (device only has I2C, so
it's kind of redundant to have i2c in the header name).

pmic_i2c.h was again not generic but designed specifically for PCA9420.
All its macros have been renamed, s/PMIC/PCA9420, and moved to the
pca9420.h header.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>