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>

Documentation/devicetree/bindings/regulator/qcom,smd-rpm-regulator.yaml

@@ -22,6 +22,8 @@ description:
   Each sub-node is identified using the node's name, with valid values listed
   for each of the pmics below.
 
+  For mp5496, s2
+
   For pm8841, s1, s2, s3, s4, s5, s6, s7, s8
 
   For pm8916, s1, s2, s3, s4, l1, l2, l3, l4, l5, l6, l7, l8, l9, l10, l11,