scsi: ufs-qcom: expose debug registers by debugfs

Add support for exposing debug registers via debugfs. This
can be used for runtime debugging of failures/errors without
the need to add more prints to the console.
In order to avoid unnecessary code duplication, the function
for dumping registers to a file (in debugfs.c) has been
slightly modified and exposed as an external function.

Signed-off-by: Dov Levenglick <dovl@codeaurora.org>
Change-Id: I77f9f3a6d041f805a93d095446f3e8077977036f
[cang@codeaurora.org: Resolved trivial merge conflicts]
Signed-off-by: Can Guo <cang@codeaurora.org>