regulator: add the sub node names for the MP5496 PMIC

MP5496 PMIC is found on IPQ6018 SoC. SMPA2 regulator controls the APSS
voltage scaling. Document the sub node name for the same.

Signed-off-by: Kathiravan T <kathirav@codeaurora.org>
Link: https://lore.kernel.org/r/1596098964-19878-3-git-send-email-kathirav@codeaurora.org


Signed-off-by: Mark Brown <broonie@kernel.org>