regmap: Support some more block operations on cached devices

Commit 10a08d9f ("regmap: Support some block operations on cached devices")
allowed raw read operations without throwing a warning when using caches if
all registers are volatile. This patch does the same for raw write operations.

This is for example useful when loading a firmware in a predefined volatile
region on a chip where we otherwise want registers to be cached.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>