ath79: add support for Atheros AR934x HS UART

AR934x chips also got the 'old' qca,ar9330-uart in addition to the
'new' ns16550a compatible one. Add support for UART1 clock selector as
well as device-tree bindings in ar934x.dtsi to make use of that uart.

Reported-by: Piotr Dymacz <pepe2k@gmail.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>