arm: socfpga: mailbox: Support sending large mailbox command

Mailbox command which is too large to fit into the mailbox
FIFO command buffer can be sent to SDM in multiple parts.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>