GitLab

Adds RTC clock to the list of the supported clocks. This allows
stm32mp_clk_*() API functions to enable, disable and set and get
rate for the clock RTC clock.

Change-Id: I8efc3f00b1f22d1912f59d1846994e9e646d6614
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>