mfd: wm8994: Update to use maple tree register cache

mfd: wm8994: Update to use maple tree register cache

The maple tree register cache is based on a much more modern data structure
than the rbtree cache and makes optimisation choices which are probably
more appropriate for modern systems than those made by the rbtree cache. In
v6.5 it has also acquired the ability to generate multi-register writes in
sync operations, bringing performance up to parity with the rbtree cache
there.

Update the wm8994 driver to use the more modern data structure.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230713-mfd-cirrus-maple-v1-9-16dacae402a8@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>