regmap: cache: Use raw I/O to sync rbtrees if we can

This will bring no meaningful benefit by itself, it is done as a separate
commit to aid bisection if there are problems with the following commits
adding support for coalescing adjacent writes.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

diff --cc drivers/base/regmap/regcache-rbtree.c
Simple merge