iommu/arm-smmu: Reduce TLB sync timeout

We're currently waiting for up to 1 second for the TLB sync command to
finish.  Unfortunately, this is long enough to trigger a watchdog bark
since the time between the watchdog pet timer interrupt and the bark
interrupt is only 1 second.  Fix this by reducing the timeout to 500
milliseconds, which is still more than enough time for the TLB sync
command to complete.

CRs-Fixed: 929880
Change-Id: I467cd5fd9caa24b4f120a7fe8c8f57c73687006b
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>