wil6210: add pm_notify handling

Adding pm_notify to allow the following:
1. Check if suspend is allowed in an earlier stage to prevent
starting the suspend procedure in case it is not allowed
2. Notify the platform driver on the suspend request

Signed-off-by: Maya Erez <qca_merez@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>