ARM: dts: msm: add stub regulator devices for sdxprairie

Add stub regulator devices for the SMPS and LDO regulators found
on PMXPRAIRIE PMIC chips which are used on SDXPRAIRIE boards.
This ensures that consumers can make use of the regulators which
have appropriate voltage limits.

Change-Id: I47b48dc627eded624b977c345cb6511280b32963
Signed-off-by: David Collins <collinsd@codeaurora.org>