ARM: dts: msm: Add stub regulator for sdxpoorwills

Add stub regulator device nodes for SMPS and LDO
supported by PMIC pmxpoorwills. Consumers can use these
regulators to request the required voltage values.

Change-Id: Ie518a4394b57a8189065206ad26d859a18068ea5
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>