arm: pmu: Fix test_overflow_interrupt()

test_overflow_interrupt() (from arm/pmu.c) has a test that passes
because the previous test leaves the state needed to pass: the
overflow status register with the expected bits. The test (that should
fail) does not enable the PMU after mem_access_loop(), which clears
the PMCR, and before writing into the software increment register.

Fix by clearing the previous test state (pmovsclr_el0) and by enabling
the PMU before the sw_incr test.

Fixes: 4f5ef94f ("arm: pmu: Test overflow interrupts")
Reported-by: Reiji Watanabe <reijiw@google.com>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>