perf: arm_spe: Prevent module unload while the PMU is in use

When the PMU driver is built as a module, the perf expects the
pmu->module to be valid, so that the driver is prevented from
being unloaded while it is in use. Fix the SPE pmu driver to
fill in this field.

Cc: Will Deacon <will.deacon@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>