Load/store/move double sub-extension

[christian-herber-nxp]

Triggered by a discussion during the internal review of Zilsd (riscv/riscv-zilsd#15), I would like to suggest the following:
The P extension will be the only extension that is not overlapping with anything and has encodings that allow defining a pair-move instruction. Zdinx also has an instruction that can be used to move a pair, but its encoding is not available when D is present.

My proposal would be for P to define a sub-extension, which contains Zilsd, Zcmlsd, and the pair move instruction.