dt: bindings: add new dt entry for pre calibration in qcom, ath10k.txt

There two things done in this patch,

1) Existing device tree entry 'qcom,ath10k-calibration-data' carries
   not only calibration data, it carries board specific data too.
   So, make appropriate update in doc.

2) ipq4019 wifi needs new devie tree entry to carry calibration
   data alone (called pre cal data, it doesn't include any other info).
   Using 'qcom,ath10k-calibration-data' for ipq4019 would alter
   the purpose of it. Hence, add new device tree entry called
   'qcom,ath10k-pre-calibration-data' to carry only pre calibration data.

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