ARM: dts: msm: add fixed regulator support for wlan_en for msm8996

The QCA wlan chipset uses tlmm, msm and pmic gpio for the WLAN_EN
signal pin. The tlmm or msm gpio uses pinctrl or gpio library for
configuring the sleep/active state runtime.

The pmic gpio runtime configuration is not feasible using gpio
library or pinctrl framework. Convert the pmic gpio to fixed
regulator to runtime control(enable/disable) via regulator framework.

CRs-Fixed: 1040537
Change-Id: I4963c2753b463d5852a847a20fe280107abca337
Signed-off-by: Sarada Prasanna Garnayak <sgarna@codeaurora.org>