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

drivers/st/clk/stm32mp1_clk.c

@@ -1094,6 +1094,7 @@ static bool clock_is_always_on(unsigned long id)
 	case CK_AXI:
 	case CK_MPU:
 	case CK_MCU:
+	case RTC:
 		return true;
 	default:
 		return false;