Add support for ARM System Control and Management Interface (SCMI) v3.1.

The SCMI specification describes a set of standard interfaces for power,
performance and system management.

SCMI is extensible and provides interfaces to access functions which are
often implemented in firmwares in the System Control Processor (SCP).

This implements Shared Memory-based transfer, which is one of the ways on
how messages are exchanged between agents and the platform.

This includes a driver for ARM Message Handling Unit (MHU) Doorbell, which
is a mechanism that the caller can use to alert the callee of the presence
of a message.

The support implements clock management interface. For instance this allows
us to control HDMI pixel clock on ARM Morello Board.

Tested on ARM Morello Board.

Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D37316
Reviewed by: manu
Sponsored by: UKRI