rtw88: add a debugfs entry to dump coex's info

Add a new entry "coex_info" in debugfs to dump coex's states for
us to debug on coex's issues.

The basic concept for co-existence (coex, usually for WiFi + BT)
is to decide a strategy based on the current status of WiFi and
BT. So, it means the WiFi driver requires to gather information
from BT side and choose a strategy (TDMA/table/HW settings).

Althrough we can easily check the current status of WiFi, e.g.,
from kernel log or just dump the hardware registers, it is still
very difficult for us to gather so many different types of WiFi
states (such as RFE config, antenna, channel/band, TRX, Power
save). Also we will need BT's information that is stored in
"struct rtw_coex". So it is necessary for us to have a debugfs
that can dump all of the WiFi/BT information required.

Note that to debug on coex related issues, we usually need a
longer period of time of coex_info dump every 2 seconds (for
example, 30 secs, so we should have 15 times of coex_info's
dump).

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Reviewed-by: Chris Chiu <chiu@endlessm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200313033008.20070-2-yhchuang@realtek.com