MIPS: Add RTC support for Loongson1C board

The patch adds RTC support for Loongson1C board, and enable the external
crystal when the RTC is first powered up.

Signed-off-by: Yang Ling <gnaygnil@gmail.com>
Cc: keguang.zhang@gmail.com
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14214/


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