The Statistical Profiling Extension (SPE) is an architectural feature we
can safely detect at runtime. However it still relies on one piece of
platform-specific information: the interrupt line it is connected
to. This requires SPE to be described in a devicetree node.

Since SPE support varies with the CPU cores found on an FPGA image, we
should detect the presence of SPE at runtime, and remove a potentially
existing SPE PMU node from the DT.

This allows to always have the SPE node in a generic devicetree file,
without risking exposing it on a CPU without this feature.

Change-Id: I73d83ea8509b03fe7bba20b9cce8d1335035fa31
Signed-off-by: Andre Przywara <andre.przywara@arm.com>