rtc: Remove the Loongson-1 RTC driver

Remove the ls1x-rtc driver as it is obsolete. We will continue to
support the ls1x RTC in the upcoming Loongson unified RTC driver
rtc-loongson.

Cc: Keguang Zhang <keguang.zhang@gmail.com>
Cc: zhao zhang <zhzhl555@gmail.com>
Cc: Yang Ling <gnaygnil@gmail.com>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Acked-by: Keguang Zhang <keguang.zhang@gmail.com>
Link: https://lore.kernel.org/r/c38c666015a162d7031b20a48209ce577bab62cd.1685693501.git.zhoubinbin@loongson.cn


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>