regmap: Don't generate gather writes for single register raw writes

regmap: Don't generate gather writes for single register raw writes

Since it is quite common for single register raw or async writes to be
generated by rbtree cache syncs or firmware downloads and essentially all
hardware will be faster with only a single transfer optimise this case by
copying single values into the internal scratch buffer before sending.

Signed-off-by: Mark Brown <broonie@linaro.org>