EDAC/cortex: Run read_errors() in parallel when checking for errors

Using smp_call_function_many() in cortex_arm64_edac_check() means the
calling CPU spends less time waiting.

Add locks around calls into edac, which only happen when an error is found,
and around the L2 accesses to prevent double counting.

Signed-off-by: James Morse <james.morse@arm.com>