ath10k: support dev_coredump for crash dump

Whenever firmware crashes, and both CONFIG_ATH10K_DEBUGFS and
CONFIG_ALLOW_DEV_COREDUMP are enabled, dump information about the crash via a
devcoredump device. Dump can be read from userspace for further analysis from:

/sys/class/devcoredump/devcd*/data

As until now we have provided the firmware crash dump file via fw_crash_dump
debugfs keep it still available but deprecate and a warning print that the user
should switch to using dev_coredump.

Future improvement would be not to depend on CONFIG_ATH10K_DEBUGFS, as there
might be systems which want to get the firmware crash dump but not enable
debugfs. How to handle memory consumption is also something which needs to be
taken into account.

Signed-off-by: Arun Khandavalli <akhandav@qti.qualcomm.com>
[kvalo@qca.qualcomm.com: rebase, fixes, improve commit log]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>