regmap: Allow empty read/write_flag_mask

All zero read and write masks in the regmap config are used to signal no
special mask is needed and the bus defaults are used. In some devices
all zero read/write masks are the special mask and bus defaults should
not be used. To signal this a new variable is added.

For example SPI often sets bit 7 in address to signal to the device a
read is requested. On TI AFE44xx parts with SPI interfaces no bit
needs to be set as registers are either read or write only and the
operation can be determined from the address only. For this case both
masks must be zero to not effect the address.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>