dt-bindings: clocks: add APB RTC gate for SC9860

Added index of RTC gate clocks which are used by some devices on aon
area of SC9860, for example the Watchdog timer.

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