lib: arm64: gic-v3-its: Add wmb() barrier before INT command

The ITS tests use the INT command like an SGI. The its_send_int() function
kicks a CPU and then the test checks that the interrupt was observed as
expected in check_lpi_stats(). This is done by using lpi_stats.observed and
lpi_stats.expected, where the target CPU only writes to lpi_stats.observed,
and the source CPU reads it and compares the values with
lpi_stats.expected.

The fact that the target CPU doesn't read data written by the source CPU
means that we don't need to do inter-processor memory synchronization
for that between the two at the moment.

The acked array is used by its-pending-migration test, but the reset value
for acked (zero) is the same as the initialization value for static
variables, so memory synchronization is again not needed.

However, that is all about to change when we modify all ITS tests to use
the same functions as the IPI tests. Add a write memory barrier to
its_send_int(), similar to the gicv3_ipi_send_mask(), which has similar
semantics.

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