mailbox: add Axiado AX3005 mailbox driver

Add a mailbox controller driver for the Axiado AX3005 SoC.
The controller provides communication channels between
the host CPU and the coprocessor.

The hardware provides 8 TX channels and 8 RX channels
through separate register regions. RX channels use
per-channel interrupts, while TX completion is detected by
polling the FIFO status.

Add the driver path to the existing Axiado mailbox entry in
MAINTAINERS.

Signed-off-by: Swark Yang <syang@axiado.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>