slimbus: Add messaging APIs to slimbus framework

SLIMbus devices use value-element, and information elements to
control device parameters (e.g. value element is used to represent
gain for codec, information element is used to represent interrupt
status for codec when codec interrupt fires).
Messaging APIs are used to set/get these value and information
elements. SLIMbus specification uses 8-bit "transaction IDs" for
messages where a read-value is anticipated. Framework uses a table
of pointers to store those TIDs and responds back to the caller in
O(1).
Caller can do synchronous and asynchronous reads/writes.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Reviwed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>