scmi: Introduce a new SCMI API and port CLK SCMI driver to it

Expose new scmi_buf_get/put API methods to build and send messages;
command request descriptors are now pre-allocated when the SCMI core is
initialized and kept in a free list, instead of being allocated on the
stack of the caller of the SCMI request.

Dynamically allocated descriptors enable the SCMI core to keep around
and track outstanding transactions for as long as needed, outliving the
lifetime of the caller stack: this allows tracking of late or missing
replies and it will be needed when adding support for SCMI transports
that allows for more messages to be inflight concurrently.

Move the existing CLK SCMI driver to the new API.

Reviewed by: andrew
Tested on: Arm Morello Board
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43046

sys: Remove $FreeBSD$: two-line .h pattern

Remove /^\s*\*\n \*\s+\$FreeBSD\$$\n/

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