ARM: dts: msm: add stub regulator devices for sdm855

Add stub regulator devices for the SMPS, LDO, and BoB regulators found
on the PM855 and PM855L PMIC chips which are used on sdm855 boards.
This ensures that consumers can make use of the regulators which have
appropriate voltage limits.

Change-Id: Ib622812ce56d03cad2440687863b7d2f3105efdd
Signed-off-by: Harry Yang <harryy@codeaurora.org>