boards/arm/mercury_xu: change board vendor to Enclustra

This commit changes the vendor of this board from Xilinx to Enclustra. It
also adds the `enclustra` vendor prefix to avoid potential name collisions
in the future.

See Enclustra Mercury XU and Mercury+ XU SoMs:
https://www.enclustra.com/en/products/system-on-chip-modules/.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>

boards/enclustra/mercury_xu/mercury_xu.dts

@@ -10,7 +10,7 @@
 
 / {
 	model = "Mercury XU";
-	compatible = "xlnx,zynqmp";
+	compatible = "enclustra,mercury_xu";
 
 	chosen {
 		zephyr,console = &uart0;

boards/enclustra/mercury_xu/mercury_xu.yaml

@@ -6,4 +6,4 @@ arch: arm
 toolchain:
   - zephyr
   - gnuarmemb
-vendor: xlnx
+vendor: enclustra