MIPS: ath25: add UART support

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: Linux MIPS <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/8242/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>