doc: releases: Update migration guilde for ADC driver

Update the migration guilde for ADC driver when support 16-bit resolution

Signed-off-by: Thinh Le Cong <[email protected]>

Author: Thinh Le Cong

doc/releases/migration-guide-4.3.rst

@@ -58,6 +58,17 @@ Device Drivers and Devicetree
 
 .. zephyr-keep-sorted-start re(^\w)
 
+ADC
+===
+
+* The :dtcompatible:`renesas,ra-adc` compatible has been replaced by
+  :dtcompatible:`renesas,ra-adc12`. Applications using the old compatible
+  must update their devicetree nodes.
+
+* The :dtcompatible:`renesas,ra-adc16` compatible was added. This must be
+  used when working with the EK-RA2A1 board, which provides a 16-bit ADC
+  resolution.
+
 MFD
 ===