nRF: add option to manually define `BT_CTLR_DEBUG_PINS`

[diggit]

Is your feature request related to a problem? Please describe.

BT_CTLR_DEBUG_PINS is currently supported only on several devboards with hardcoded pins. This approach makes it impossible to use this feature on custom boards.

Describe the solution you'd like

Use similar approach to Bluetooth CS antenna switching GPIO definition via DT overlay.

Describe alternatives you've considered

Injection of custom header could be also option.

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