ARM: perf: add devicetree bindings for 11MPcore, A5, A7 and A15 PMUs

This patch adds separate devicetree bindings for 11MPcore and
Cortex-{A5,A7,A15} PMUs in preparation for improved devicetree parsing
in the ARM perf-event CPU PMU driver.

Cc: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>