ARM: 7584/1: perf: fix link error when CONFIG_HW_PERF_EVENTS is not selected

Commit e50c5418 (ARM: perf: add guest vs host discrimination) broken the
link as perf_instruction_pointer and perf_misc_flags are not defined
when CONFIG_HW_PERF_EVENTS is not selected.

As it make little sense to try and profile a guest without any HW event,
just fallback to the original code when this config option is not selected.

Reported-by: Russell King <linux@arm.linux.org.uk>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>