clk: stm32mp1: merge 'ck_hse_rtc' and 'ck_rtc' into one clock

clk: stm32mp1: merge 'ck_hse_rtc' and 'ck_rtc' into one clock

'ck_rtc' has multiple clocks as input (ck_hsi, ck_lsi, and ck_hse).
A divider is available only on the specific rtc input for ck_hse.
This Merge will facilitate to have a more coherent clock tree
in no trusted / trusted world.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Link: https://lore.kernel.org/r/20210617051814.12018-3-gabriel.fernandez@foss.st.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>