dts: arm: st: wb: Add RTC BBRAM to DTS

All STM32WB devices have 20 4-byte battery-backed backup registers in
RTC.

Signed-off-by: Patryk Duda <[email protected]>

Author: duda-patryk

dts/arm/st/wb/stm32wb.dtsi

@@ -247,6 +247,12 @@
 			clocks = <&rcc STM32_CLOCK_BUS_APB1 0x00000400>;
 			prescaler = <32768>;
 			status = "disabled";
+
+			bbram: backup_regs {
+				compatible = "st,stm32-bbram";
+				st,backup-regs = <20>;
+				status = "disabled";
+			};
 		};
 
 		spi1: spi@40013000 {