MIPS: Fix always CONFIG_LOONGSON_UART_BASE=y

Signed-off-by: Yoichi Yuasa <yuasa@linux-mips.org>
Cc: linux-mips <linux-mips@linux-mips.org>
Patchwork: https://patchwork.linux-mips.org/patch/2055/


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