ARM: dts: msm: add PM8994 LDO 18 pin control regulator for msm8996

Add a PM8994 LDO 18 regulator device which can be used to manage
the HW_EN2 pin control enable configuration for LDO 18 at
runtime.  When this regulator is enabled, LDO 18 output is
enabled whenever HW_EN2 (i.e. PM8994 GPIO 11) is driven high.
When this regulator is disabled, the state of GPIO 11 is ignored.

This regulator can be used to support WLAN antenna sharing where
the WLAN chip is connected to GPIO 11 and drives it high whenever
RX or TX is active.

Also update the voltage constraint range to allow values from
2.7 V to 2.9 V.

Change-Id: I7afab38356e6619580539ed05b5130553491b100
Signed-off-by: David Collins <collinsd@codeaurora.org>