feat(mhu): add MHUv3 doorbell driver

MHUv3 reworks parts of MHUv2 and introduces MHU extensions. There are
currently 3 extensions:

* Doorbell extension: which works like MHUv2
* FIFO extension: which uses a buffer for faster inband data transfer
* Fastchannel extension: for fast data transfer

Add MHUv3 driver with support for Doorbell extension for both postbox
sender MHUs and mailbox receiver MHUs.

Signed-off-by: Aziz IDOMAR <aziz.idomar@arm.com>
Signed-off-by: Sayanta Pattanayak <sayanta.pattanayak@arm.com>
Signed-off-by: Shriram K <shriram.k@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Signed-off-by: Joel Goddard <joel.goddard@arm.com>
Change-Id: Icf49df56f1159f4c9830e0ffcda5b3a4bea8d2fd
3 files changed