i3c: Add sysfs ABI spec

Document sysfs files/directories/symlinks exposed by the I3C subsystem.

Change-Id: I9865931b024640ac84174cd5e4dd937ccc0b5468
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Git-commit: a9f3edd0dd876bb9d39dec1dea90821460631ce4
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Alok Chauhan <alokc@codeaurora.org>