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>