drivers/rtc/rtc_mc146818.c: Add input clock config to driver

This commit adds input clock selection to the RTC driver. This
is required to allow for the real hardware to operate. The
QEMU emulated hardware ignores the input clock settings.

Signed-off-by: Bjarki Arge Andreasen <bjarkix123@gmail.com>

dts/bindings/rtc/motorola,mc146818.yaml

@@ -7,3 +7,13 @@ description: Motorola MC146818 compatible Real Timer Clock
 compatible: "motorola,mc146818"
 
 include: rtc-device.yaml
+
+properties:
+  clock-frequency:
+    type: int
+    default: 32768
+    enum:
+      - 4194304
+      - 1048576
+      - 32768
+    description: Frequency of the input-clock in Hertz (Hz)