dt-bindings: regulator: rpmh-regulator: add SVS_L2 voltage level

Add a definition for RPMH_REGULATOR_LEVEL_SVS_L2.  This is needed
to support the VDD_GFX voltage level for a new intermediate GPU
frequency on SM8150 boards.

Change-Id: Ie4cbeb2f6f4c65f87f89005d32a7f7eecf710199
Signed-off-by: David Collins <collinsd@codeaurora.org>
Signed-off-by: Kyle Piefer <kpiefer@codeaurora.org>