net: ravb: Move reference clock enable/disable on runtime PM APIs

Reference clock could be or not be part of the power domain. If it is part
of the power domain, the power domain takes care of properly setting it. In
case it is not part of the power domain and full runtime PM support is
available in driver the clock will not be propertly disabled/enabled at
runtime. For this, keep the prepare/unprepare operations in the driver's
probe()/remove() functions and move the enable/disable in runtime PM
functions.

By doing this, the previous ravb_runtime_nop() function was renamed
ravb_runtime_suspend() and the comment was removed. A proper runtime PM
resume function was added (ravb_runtime_resume()). The current driver
still don't need to make any register settings on runtime suspend/resume
(as expressed in the removed comment) because, currently,
pm_runtime_put_sync() is called on the driver remove function. This will be
changed in the next commits (that extends the runtime PM support) such
that proper register settings (along with runtime resume/suspend) will be
done on ravb_open()/ravb_close().

Along with it, the other clock request operations were moved close to
reference clock request and prepare to have all the clock requests
specific code grouped together.

Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>