rtio: Add iodev specific flags

Flags are needed for i2c support to enable arbitrary stop and restart
setups where in a single i2c transaction the caller would like to direct
the hardware to either send a stop or not after any particular read/write.

These can be reused more generally to direct devices to use different
methods of performing the operations on an operation by operation basis
such as perhaps a hint to use a polling, interrupt, or dma read/write.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>