Add PM handlers for System suspend/resume.

As DMA driver doesn't yet support suspend/resume we free up
the DMA channels at suspend and acquire and initialize them
at resume.

In this revised approach we do not free the TX/RX IRQs at
am65_cpsw_nuss_common_stop() as it causes problems.
We will now free them only on .suspend() as we need to release
the DMA channels (as DMA looses context) and re-acquiring
them on .resume() may not necessarily give us the same
IRQs.

To make this easier:
- introduce am65_cpsw_nuss_remove_rx_chns() which is
   similar to am65_cpsw_nuss_remove_tx_chns(). These will
   be invoked in pm.suspend() to release the DMA channels
   and free up the IRQs.
- move napi_add() and request_irq() calls to
   am65_cpsw_nuss_init_rx/tx_chns() so we can invoke them
   in pm.resume() to acquire the DMA channels and IRQs.

As CPTS looses contect during suspend/resume, invoke the
necessary CPTS suspend/resume helpers.

ALE_CLEAR command is issued in cpsw_ale_start() so no need
to issue it before the call to cpsw_ale_start().
Signed-off-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>