serdev: add dev_pm_domain_attach|detach()

In order to open up the required power gate before any operation can be
effectively performed over the serial bus between CPU and serdev, it's
clearly essential to add common attach functions for PM domains to serdev
at the probe phase.

Similarly, the relevant dettach function for the PM domains should be
properly and reversely added at the remove phase.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>