perf/arm-cmn: Reduce stack usage during discovery

Arnd reports that Clang's aggressive inlining of arm_cmn_discover() can
lead to stack frame size warnings, and while we could simply prevent
such inlining to hide the issue, it seems more productive to actually
heed the warning and do something about the overall stack footprint.
The xp_region array is already rather large, and CMN_MAX_XPS might only
grow larger in future, however it only serves as a convenience to save
repeating the first level's worth of register reads in the second pass
of discovery. There's no performance concern here, and it only takes a
small tweak to the flow to re-extract the offsets instead of stashing
them, so let's just do that and save several hundred bytes of stack.

Reported-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>