fix(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
diff --git a/drivers/st/clk/stm32mp1_clk.c b/drivers/st/clk/stm32mp1_clk.c
index a341cdb..3ebc376 100644
--- a/drivers/st/clk/stm32mp1_clk.c
+++ b/drivers/st/clk/stm32mp1_clk.c
@@ -1094,6 +1094,7 @@
 	case CK_AXI:
 	case CK_MPU:
 	case CK_MCU:
+	case RTC:
 		return true;
 	default:
 		return false;