scmi: mailbox: Increase message slots

commit 228d98343155e527dee6a0e6f8ba6ffd8669fe41 from
git@git.assembla.com:cavium/WindRiver.linux.git

Increasing the max_msg and MBOX_TX_QUEUE_LEN to increase the
message slot to support CN10k 24 cores.

Signed-off-by: Piyush Malgujar <pmalgujar@marvell.com>
Change-Id: Ia0fb21b15a56bf224ace09652dbfd3ed724ce1ae
Reviewed-on: https://sj1git1.cavium.com/c/IP/SW/kernel/linux/+/99330


Reviewed-by: Chandrakala Chavva <cchavva@marvell.com>
Reviewed-by: Jayanthi Annadurai <jannadurai@marvell.com>
Tested-by: sa_ip-sw-jenkins <sa_ip-sw-jenkins@marvell.com>
Signed-off-by: Ruiqiang Hao <Ruiqiang.Hao@windriver.com>