staging: fsl-mc/dpio: Add dpaa2_io_service_select() API

All DPIO service API functions receive a dpaa2_io service pointer
as parameter (NULL meaning any service will do) which indicates
the hardware resource to be used to execute the specified command.

There isn't however any available API for obtaining such a service
reference that could be used further, effectively forcing the users
to always request a random service for DPIO operations.
(The DPIO driver holds internally an array mapping services to cpus,
and affine services can be indirectly requested by a couple of API
functions: dpaa2_io_service_register and dpaa2_io_service_rearm
use the cpu id provided by the user to select the corresponding
service)

This patch adds a function for selecting a DPIO service based on
the specified cpu id. If the user provides a "don't care" value
for the cpu, we revert to the default behavior and return the next
DPIO, taken in a round-robin fashion from a list of available
services.

Signed-off-by: Ioana Radulescu <ruxandra.radulescu@nxp.com>
Acked-by: Roy Pledge <roy.pledge@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>