Add serial feature to Nucleo F746ZG

[ghost]

Description

At the moment, only UART2 and UART6 enabled.

How Has This Been Tested?

Tested on UART2 with a serial display.

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[nfbot]

Hi @MikroBusNet,

I'm nanoFramework bot.
Thank you for your contribution!

A human will be reviewing it shortly. 😉