ath79: ar933x_uart: set UART_CS_{RX,TX}_READY_ORIDE

ath79: ar933x_uart: set UART_CS_{RX,TX}_READY_ORIDE

On AR934x this UART is usually not initialized by the bootloader
as it is only used as a secondary serial port while the primary
UART is a newly introduced NS16550-compatible.
In order to make use of the ar933x-uart on AR934x without RTS/CTS
hardware flow control, one needs to set the
UART_CS_{RX,TX}_READY_ORIDE bits as other than on AR933x where this
UART is used as primary/console, the bootloader on AR934x typically
doesn't set those bits.
Setting them explicitely on AR933x does not do any harm, so just set
them unconditionally.

Tested-by: Chuanhong Guo <gch981213@gmail.com
Signed-off-by: Daniel Golle <daniel@makrotopia.org>