Alchemy: Add UART PM methods.

Custom UART PM hook for Alchemy chips:  do standard UART pm and
additionally en/disable uart block clocks as needed.
This allows to get rid of a debug port PM hack in the Alchemy pm code.

Tested on Db1200.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>