samples/drivers/adc/adc_sequence: Build fix for sam_v71_xult

Fix build error due to cmake requiring a separate
overlay for each of this target's SOCs.

Error being fixed:
```
-- Board: sam_v71_xult, qualifiers: samv71q21
CMake Error at cmake/modules/extensions.cmake:2742 (message):
Board sam_v71_xult defines multiple SoCs.

Shortened file name (sam_v71_xult.overlay) not allowed, use
'<board>_<soc>.overlay' naming
```

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>