dt-bindings: riscv: isa string bonus content

**NOT FOR CONSIDERATION**

I figured, sure why not add the strings for version number validation,
just in case we need them in the future. The commented out string is
considered by dt-schema to be "not a regex", but regex101 thinks it
is... Maybe dt-schema does not support named match groups, but they
are the only way that I could trivially find to make this somewhat
manageable. Either way, it is permissive so it allows combinations
of "M", "MpM" & no number.

Not-signed-off-by: Conor Dooley <conor.dooley@microchip.com>
1 file changed