gpio: fx6408: Convert 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.

Signed-off-by: Mark Brown <[e-mail]>
Reviewed-by: Andy Shevchenko <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>

drivers/gpio/gpio-fxl6408.c

···
 	.rd_table = &rd_table,
 	.volatile_table = &volatile_table,
 
-	.cache_type = REGCACHE_RBTREE,
+	.cache_type = REGCACHE_MAPLE,
 	.num_reg_defaults_raw = FXL6408_REG_INT_STS + 1,
 };