rtio: Introduce OP_DELAY as a valid SQE operation

SQE items with this OP will take the specified amount of time
(asynchronously) before completing. This allows to serve as an
asynchronous delay in between SQE items (e.g: A sensor measurement
requested, which requires 50-ms before having the result available).

Signed-off-by: Luis Ubieda <luisf@croxel.com>