dt-bindings: power: rpmpd: Add MSM8226 to rpmpd binding

Add compatible and constants for the power domains exposed by the
MSM8226 RPM.

Signed-off-by: Luca Weiss <luca@z3ntu.xyz>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220220223004.507739-1-luca@z3ntu.xyz