m68k: cascade m68k_features by m680xx_cpu_initfn() to improve readability

The m680XX_cpu_initfn functions have been rearranged to cascade starting from
the base 68000, so that the 68010 then inherits from this, and so on until the
68060.

This makes it simpler to track features since in most cases the m68k were
product enhancements on each other, with only a few instructions being retired.

Because each cpu class inherits the previous CPU class, then for example
the 68020 also has the feature 68010, and 68000 and so on upto the 68060.

- Added 68010 cpu class, and moved correct features into 68000/68010.
- Added m68k_unset_feature to allow removing a feature in the inheritence

Signed-off-by: Lucien Murray-Pitts <lucienmp.qemu@gmail.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <c652fe7537f8b4fe87a13ecbbc0ea751fb71532f.1612137712.git.balaton@eik.bme.hu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>