mailbox: arm_mhu: reduce txpoll_period from 10ms to 1 ms

Since the mailbox core users hrtimers now, it can handle much higher
resolutions. We can reduce the txpoll_period to 1 ms as the transmit
usually takes just few microseconds.

Reported-and-suggested-by: Juri Lelli <Juri.Lelli@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>