ath10k/WCN3990: move wlanmdsp to qcom/sdm845

ath10k/WCN3990: move wlanmdsp to qcom/sdm845

The firmware file ath10k/WCN3990/hw1.0/wlanmdsp.mbn is platform-specific
(rather than being specific to the WiFi chip itself) and can be used
only on the Qualcomm SDM845 platform and close relatives. Other
platforms (e.g. Qualcomm QCM2290, QRB4210) are going to provie their own
copies of the wlanmdsp.mbn firmware.

As discussed with Kalle, move this file to the platform-specific folder,
providing the backwards-compatibility symlink.

Cc: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Josh Boyer <jwboyer@kernel.org>