dt: bindings: add new dt entry for ath10k calibration variant

The bus + bmi-chip-id + bmi-board-id is not enough to identify the correct
board data file on QCA4019 based devices. Multiple different boards share
the same values. Only the original reference designs can currently be
identified and loaded from the board-2.bin. But these will not result in
the correct calibration data when combined with the pre-calibration data
from the device.

An additional "variant" information has to be provided (via SMBIOS or DT)
to select the correct board data for a design which was modified by an ODM.

Signed-off-by: Sven Eckelmann <sven.eckelmann@openmesh.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>