rtc: Add rtc driver for the Loongson family chips

The Loongson family chips use an on-chip counter 0 (Time Of Year
counter) as the RTC. We will refer to them as rtc-loongson.

Cc: Keguang Zhang <keguang.zhang@gmail.com>
Cc: Yang Ling <gnaygnil@gmail.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@kernel.org>
Signed-off-by: WANG Xuerui <git@xen0n.name>
Reviewed-by: Keguang Zhang <keguang.zhang@gmail.com>
Tested-by: Keguang Zhang <keguang.zhang@gmail.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com> # LS7A
Link: https://lore.kernel.org/r/0c5171156390f614d72f36ceb04a20f432ca639e.1685693501.git.zhoubinbin@loongson.cn
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>