clk: sprd: add RTC gate for SC9860

Add a few gate clocks which are used for gating RTC for some
devices on AON area of SC9860.

This patch has been tested on SC9860, with this patch and proper DT
configurations, the watchdog can be initialized and work well.

Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>