msm: msm_bus: Add device tree support for bus scaling clients

This patch enables clients to specify vectors in device tree
format. Clients will no longer need to specify vectors in
the boards file. Instead, they can add the bus-scaling
related data to their own device-nodes.
The clients can then call an API exposed by the bus driver
which will build the pdata out of those strings.
The registration process using pdata remains unchanged.

Change-Id: I1bba645d031e2cd822f16d8edcdf926a0418618c
Signed-off-by: Gagan Mac <gmac@codeaurora.org>