mtip32xx: Add debugfs entry device_status

This patch adds a new debugfs entry 'device_status' in
/sys/kernel/debug/rssd. The value of this entry shows
devices online and those in the process of removing.

Signed-off-by: Sam Bradshaw <sbradshaw@micron.com>
Signed-off-by: Asai Thambi S P <asamymuthupa@micron.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>