coresight: stm: Remove spin lock usage for channel allocation

Current usage of spinlock in channel allocation might result in
live lock issue. In order to avoid this we move to a per cpu
channel allocation scheme. We rely on preemption being disabled
when we allocate channel to avoid any conflict. If we end up
handling an interrupt which logs when preemption is disabled it
is still gauranteed to not interfere with the current logging
as the channel id will be freed up before resuming.

Change-Id: I91d7fcb4ed9202328e62d5459283927e238f9f66
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>