m68k: improve comments on m68k_move_to/from helpers

Add more detailed comments to each case of m68k_move_to/from helpers to list
the supported CPUs for that CR as they were wrong in some cases, and
missing some cpu classes in other cases.

Signed-off-by: Lucien Murray-Pitts <lucienmp.qemu@gmail.com>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <a8bd70b66e3dbdb7d2ab7a852af71cdbf341d50c.1612137712.git.balaton@eik.bme.hu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>