arm_fpga: Remove SPE PMU DT node if SPE is not available

arm_fpga: Remove SPE PMU DT node if SPE is not available

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>