regmap: Split use_single_rw internally into use_single_read/write

regmap: Split use_single_rw internally into use_single_read/write

use_single_rw currently reflects the capabilities of the connected
device. The capabilities of the bus are currently missing for this
variable.

As there are read only and write only buses we need seperate values for
use_single_rw to also reflect tha capabilities of the bus.

This patch splits use_single_rw into use_single_read and
use_single_write. The initialization is changed to check the
configuration for use_single_rw and to check the capabilities of the
used bus.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>