s3cmci: add debugfs support for examining driver and hardware state

Export driver state and hardware register state via debugfs entries
created under a directory formed from dev_name() on the probed device when
CONFIG_DEBUG_FS is set.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>