regmap: add reg_read/reg_write callbacks to regmap_bus struct

Some busses do not support sending/receiving multiple registers in one go.
Such kind of busses just unpack the registers that have been previously
packed by the regmap core or pack registers that will be later unpacked by
the core code.

Add reg_write and reg_read callbacks in order to optimize access through
this kind of busses.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Signed-off-by: Mark Brown <broonie@linaro.org>