coresight: add checks to validate etm trace unit resource values

Hardware sets the register values to indicate how many resources the trace
unit supports. Add checks to validate these values to avoid any out of
bounds access.

Change-Id: I5f19a1351af3bc727f6fb7743a258727fa80fc06
Signed-off-by: Neeti Desai <neetid@codeaurora.org>
Signed-off-by: Sarangdhar Joshi <spjoshi@codeaurora.org>