regulator: rpmh-regulator: add header for voltage levels

Add a header file for the rpmh-regulator voltage level constants
to be used in device tree regulator configurations.

Change-Id: I63138e712ed8da693a7107a6999e8c7d70dbf8c4
CRs-Fixed: 1055890
Signed-off-by: David Collins <collinsd@codeaurora.org>