DT compatible string versioning for SiFive open-source IP blocks

This document describes the version specification for DT "compatible"
strings for open-source SiFive IP blocks.  HDL for these IP blocks
can be found in this public repository:

https://github.com/sifive/sifive-blocks

IP block-specific DT compatible strings are contained within the HDL,
in the form "sifive,<ip-block-name><integer version number>".

An example is "sifive,uart0" from:

https://github.com/sifive/sifive-blocks/blob/v1.0/src/main/scala/devices/uart/UART.scala#L43

Until these IP blocks (or IP integration) support version
auto-discovery, the maintainers of these IP blocks intend to increment
the suffixed number in the compatible string whenever the software
interface to these IP blocks changes, or when the functionality of the
underlying IP blocks changes in a way that software should be aware of.

Driver developers can use compatible string "match" values such as
"sifive,uart0" to indicate that their driver is compatible with the
register interface and functionality associated with the relevant
upstream sifive-blocks commits.  It is expected that most drivers will
match on these IP block-specific compatible strings.

DT data authors, when writing data for a particular SoC, should
continue to specify an SoC-specific compatible string value, such as
"sifive,fu540-c000-uart".  This way, if SoC-specific
integration-specific bug fixes or workarounds are needed, the kernel
or other system software can match on this string to apply them.  The
IP block-specific compatible string (such as "sifive,uart0") should
then be specified as a subsequent value.

An example of this style:

    compatible = "sifive,fu540-c000-uart", "sifive,uart0";