MIPS: ath79: register UART for the QCA955X SoCs

Similarly to the preceding SoCs, the QCA955X SoCs
also have a built-in NS16650 compatible UART.
Register the platform device for that to make
it usable.

Cc: Rodriguez, Luis <rodrigue@qca.qualcomm.com>
Cc: Giori, Kathy <kgiori@qca.qualcomm.com>
Cc: QCA Linux Team <qca-linux-team@qca.qualcomm.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/4949/
Signed-off-by: John Crispin <blogic@openwrt.org>