regmap: slimbus: add support to multi read/write

SLIMbus supports upto 16 bytes in value management messages,
so add support to read/writes upto 16 bytes.

This also removes redundant single register reg_read/reg_write.

Also useful for paged register access on SLIMbus interfaced codecs.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>