clk: Ingenic: Add RTC related clocks for Ingenic SoCs.

The RTC unit in the Ingenic SoCs has two clock sources, one
is from an external 32.768kHz clock, and the other is from an
external 24MHz/48MHz main clock that is divided by 512. The
choice of these two clocks is controlled by the ERCS bit in
the OPCR register. The RNG unit will also use this clock.

Tested-by: 周正 (Zhou Zheng) <sernia.zhou@foxmail.com>
Signed-off-by: 周琰杰 (Zhou Yanjie) <zhouyanjie@wanyeetech.com>
Link: https://lore.kernel.org/r/20200725051136.58220-4-zhouyanjie@wanyeetech.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>