machine:tanaro: Added RTC overlays

In the future, the TANARO will be produced with two possible
RTC variants. Both variants are present in the main devicetree
but are disabled by default. During the installation, one of
the RTCs will be activated via one of the two devicetree overlays.

BCS 746-000877

(cherry picked from commit df176aae)

conf/machine/imx8mguf.conf

@@ -33,6 +33,8 @@ OPTEE_BIN_EXT = "8m"
 # All available device trees for the mx8mm.
 # The first item in the list is the default DT.
 KERNEL_DEVICETREE ?= " \
+    seconorth/pcf8563-overlay.dtb \
+    seconorth/rx8803-overlay.dtb \
     seconorth/seconorth-tanaro-fg0700w0dsswagl1.dtb \
     seconorth/seconorth-tanaro-g121xcett122e.dtb \
     seconorth/seconorth-tanaro-qp_070wsvgamlli1d.dtb \