slimbus: Optimize transaction handling in the framework

Avoid overhead of allocating/freeing structure for transaction
if that transaction is synchronous. For synchronous transactions,
structure is defined on stack using a macro for simplicity. Since
structure is not allocated per transaction and locking is required
only for a few simple instructons, spinlock can be
used to protect the transaction table.

Change-Id: I79ea64b98907ab71fd727d76cf78a2d6feaf0eb0
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>