dt-binding: regulator: rpmh-regulator: define RPMh sets and regulator modes

Add #define constants for RPMh set and PMIC regulator mode
specification.  These can be used for rpmh-regulator device tree
node properties.

Change-Id: I43e50c011f38d7c0f30333b031872e8ef108d342
Signed-off-by: David Collins <collinsd@codeaurora.org>