Add variant: NUCLEO-H503RB.

[dojyorin]

Add variant: NUCLEO-H503RB

Summary

This PR fixes/implements the following bugs/features

• Add new variant NUCLEO-H503RB

@fpistm feat adds STM32H5 core implementation and NUCLEO-H563ZI support (That was really great help! Thank you!), but variant support is still lacking.

This time, I'll add new H5 NUCLEO board NUCLEO-H503RB with this pull request.
Related to #722 (comment) and #2091

Progress

There's still some work to do.

• Adapt variant_NUCLEO_H503RB.cpp and variant_NUCLEO_H503RB.h pin assignments to actual board values.
• Regarding peripheral clock supply, there are no definitions for commonly used peripherals such as I2C or SPI, is it already defined elsewhere?
• There any other missing parts?

Basically implementation of NUCLEO-H563ZI is used reference.

I'm new to STM32, so excuse me if I'm doing something wrong.
I would like to discuss with everyone and produce good results.

[fpistm]

Thanks for squashing. Anyway simply force update on the same branch will avoid creating new PR. :wink : Let ci run to check this PR first.

[fpistm]

First thanks for this PR.
In addition to my comments, could you add the generic and the Nucleo in the README.md and also declare them to the boards.txt.
Finally, split in 2 commit, 1 for the generic addition and one for the dedicated Nucleo board.

You can refers to this PR as example:
#1859

[dojyorin]

Thank you for detailed explanation, it really helped me.
It was a great reference when I wanted to add other boards in the future.

For the time being, I've fixed the part you told me, so I'll commit it.

[dojyorin]

The Arduino pinout only goes up to D15/A5, but the MCU has more pins.
I wasn't sure what rules to define the "surplus D16/A6 and beyond" pins.

[fpistm]

Hi @dojyorin
I've reworked and fixed your PR.
https://github.com/fpistm/Arduino_Core_STM32/tree/feat-nucleo-h503rb
Could you try it and force update to your branch. Thanks