msm: rtb: Convert RTB to platform device

Rather than have the size for rtb fixed in the driver,
make rtb be a platform device so the size can be passed
in as platform data. A module parameter isn't enough
since the contiguous block of memory needs to be pre-reserved.

Change-Id: I0c8f9534bb5a86f2573c85df6be7a236ff1d3691
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>