net: dsa: ar9331: Use maple tree register cache

We now have a regmap cache which uses a maple tree to store the register
state, this is a more modern data structure and the regmap level code
using it makes a number of assumptions better tuned for modern hardware
than those made by the rbtree cache type that the at9331 driver uses.
Switch the ar9331 driver to use the more modern data structure.

This should have minimal practical impact, it's mainly code
modernisation.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>