arm64: dts: qcom: db820c: Add vdd_gfx and tie it into mmcc

Add the SPMI regulator node in the PMI8994, use it to give us VDD_GX
at a fixed max nominal voltage for the db820c and specify this as supply
for the MMSS GPU_GX GDSC.

With the introduction of CPR support the range for VDD_GX should be
expanded.

Reviewed-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Rajendra Nayak <rnayak@codeaurora.org>
Link: https://lore.kernel.org/r/20200417070044.1376212-4-bjorn.andersson@linaro.org


[bjorn: Split between pmi8994 and db820c, changed voltage, rewrote commit message]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>