spmi: Debug-fs support for SPMI

Add user space interface to the SPMI kernel framework that enables
master-read and master-write transactions.

Each SPMI controller is represented as a file under the root directory
of SPMI debug-fs.  For each controller, 'address', 'count', and 'data'
files are added.  The user may configure the transaction via the
respective files and invoke a master read transaction by reading from
the 'data' file, and a master write transaction by writing a string of
decimal or hexadecimal byte size values to the 'data' file.

Change-Id: I3e760be7796bfb9aa15c8ab758ff616a72240176
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>
Signed-off-by: Kenneth Heitke <kheitke@codeaurora.org>