rpmsg: glink: Add option to affine resources to CPUs

On certain systems, all resources for a specific usecase may be pinned
to a specific cpu set to meet power and performance requirements. Add
support to define a cpu set for the irq and rx thread in device tree.
This change does not include any hotplug support.

Change-Id: I0e18e3cb1880886dddf20ce0e2744e7af3b8b553
Signed-off-by: Chris Lew <clew@codeaurora.org>