arm64: Move get_id_aa64dfr0() in processor.h

SPE support is reported in the ID_AA64DFR0_EL1 register. Move the function
get_id_aa64dfr0() from the pmu test to processor.h so it can be reused by
the SPE tests.

[ Alexandru E: Reworded commit ]

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>