firmware: arm_scmi: Increase MBOX_TX_QUEUE_LEN

Marvell-cn106xx has 24 cpu cores, each of which uses a separate policy,
so we need 24 message slot at least to ensure the normal data flow for
cpu_freq. Increase MBOX_TX_QUEUE_LEN and the max_msg of scmi_xfer to avoid
the following errors:

arm-scmi firmware:scmi: failed to get free message slot(-12)

Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>