Add device tree binding documentation details for wifi block present
in Qualcomm IPQ4019 SoC into "qcom,ath10k.txt".

Right now, ath10k wireless driver has support for PCI based wlan devices.
There is a plan to extend ath10k driver to support wifi devices which are
connected over AHB as well (enumeration will happen via device tree node).
For AHB based devices (ie, ipq4019), all wifi properties are supplied via
device tree (including irq, reg addr, cal data,etc).

Signed-off-by: Raja Mani <rmani@qti.qualcomm.com>
Acked-by: Rob Herring <robh@kernel.org>
[kvalo@qca.qualcomm.com: fixed typos noticed by Rob]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>