workqueue: Cleanup subsys attribute registration

While reviewing users of subsys_virtual_register() I noticed that
wq_sysfs_init() ignores the @groups argument. This looks like a
historical artifact as the original wq_subsys only had one attribute to
register.

On the way to building up an @groups argument to pass to
subsys_virtual_register() a few more cleanups fell out:

* Use DEVICE_ATTR_RO() and DEVICE_ATTR_RW() for
  cpumask_{isolated,requested} and cpumask respectively. Rename the
  @show and @store methods accordingly.

* Co-locate the attribute definition with the methods. This required
  moving wq_unbound_cpumask_show down next to wq_unbound_cpumask_store
  (renamed to cpumask_show() and cpumask_store())

* Use ATTRIBUTE_GROUPS() to skip some boilerplate declarations

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>