dt-bindings: mfd: qcom,qca639x: add binding for QCA639x defvice

Qualcomm QCA639x is a family of WiFi + Bluetooth SoCs, with BT part
being controlled through the UART and WiFi being present on PCIe bus.
Both blocks share common power sources. Add binding to describe power
sequencing required to power up this device.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>