clk: stm32mp1_clk: keep rtc clock always on

On battery powered systems the RTC keeps the date/time across
system reboot.
The RTC clock should not be disabled otherwise the date/time
counter gets stopped.

Tag RTC clock as always on.

Signed-off-by: HE Shushan <shushan.he@st.com>
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Change-Id: I6455c3c740d2e5add28255eb84f8ebaf2870d9d8
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/225732


Tested-by: Yann GAUTIER <yann.gautier@foss.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@foss.st.com>
Reviewed-by: CITOOLS <MDG-smet-aci-reviews@list.st.com>
Reviewed-by: Antonio Maria BORNEO <antonio.borneo@st.com>
Reviewed-by: Sebastien PASDELOUP <sebastien.pasdeloup-ext@st.com>