perf vendor events intel: Add uncore_arb JSON support

The JSON lists call the box iMPH-U, while perf calls it arb.
Add conversion support to json to convert the unit properly.

Cc: jolsa@kernel.org
Link: http://lkml.kernel.org/n/tip-stq5ly95z2qioggp9bfaqe0h@git.kernel.org
Signed-off-by: Andi Kleen <ak@linux.intel.com>