ARM: dts: msm: add hl7509 regulator device to msm8996

Halo hl7509 buck converter will be powering the VDD_GFX voltage
rail of 8996 automotive platforms. HL7509 requires an enable gpio
to turn on the external buck. Support it through a fixed
regulator.

Add them to the list of regulators for msm8996. Keep them
disabled by default so that they can be enabled in board specific
files.

CRs-Fixed: 978343
Change-Id: Ib8006e640206a92ac6f227a4a180efbe7d15026f
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>