serial: 8250_pnp: Support configurable clock frequency

ACPI-based Loongson boards need configurable rather than fixed clock
frequency for serial ports.

Signed-off-by: Jianmin Lv <lvjianmin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Link: https://lore.kernel.org/r/20210715061637.134436-1-chenhuacai@loongson.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>