scmi mailbox max_rx_timeout_ms value

[Peng Fan]

Hi Sudeep, Cristian and DT maintainers

In drivers/firmware/arm_scmi/mailbox.c, current max_rx_timeout_ms
is 30ms, we wanna to enlarge the value. NXP downstream value is set
to 1000ms, but for upstream I think it may not be a good solution
that just enlarge it for all scmi users, each platform may have its own
max timeout value depends on scmi firmware design. So I am thinking
to use a device tree property for this, saying "mbox-rx-timeout-us",
just as "atomic-threshold-us" in arm,scmi.yaml

How do you think?

Thanks,
Peng.