nanokernel: Introduce workqueue API

Add a generic API for drivers to start workqueues and submit work
items.  This is needed by drivers which need to schedule code that might
sleep from an ISR to run in fiber context.

Also add the option to start a system-wide workqueue.

Both additions are optional.  They can be deactivated for systems that
do not need them.

Change-Id: Ia843568fde5daf6d4279ef7bf241c26c1e3dcfb7
Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>