MIPS: Loongson: RTC: Enable legacy RTC driver on fulong

RTC_LIB is selected by MIPS by default, and therefore, the legacy RTC
driver is disabled. but fortunately, RTC_LIB not works on fulong, so,
enabling the legcy RTC driver is needed, otherwise, the tools like
hwclock will not work.

Signed-off-by: Wu Zhangjin <wuzj@lemote.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>