rockchip: px30: Rename CONFIG_DEBUG_UART2_CHANNEL to CONFIG_DEBUG_UART_CHANNEL

UART3 also has two sets of pins that can be selected.

Rename the config option to a common name, to allow it to be used for both
UART2 and UART3.

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>