ARM: dts: add PM8004 dual phase SMPS2/SMPS4 regulator node for VDD_GFX

The dual phase SMPS2/SMPS4 regulator will be used to supply VDD_GFX
on msm8994 platforms which use PM8004.

Add the relevant rpm-regulator nodes which are referred to as smpc2.
This allows consumers to vote for the respective floor corner and
corner voltages.

Change-Id: Icdb97ffe5dd023c3ac86653964524b03f968fe9a
Signed-off-by: David Keitel <dkeitel@codeaurora.org>