arch/xmc4 vadc driver - background request source partial support

[adriendesp]

Summary

Added driver for XMC4[78] and XMC45 series VADC peripheral. Driver includes :

• Complete register map and bit fields detail for both series.
• VADC Group and Channel config handler for easy register programming.
• Background Scan request source partial support but working conversion with basic configuration.

Impact

Testing

Tested on a XMC4800 custom board with 3 channels of same group.