boards/stm32f7508-dk: add support

[aabadie]

Contribution description

This PR adds support for the stm32f7508-dk board. Features wise it's exactly the same board as the stm32f746g-disco, the only difference is the microcontroller model. This one has an stm32f750n8 with much less space on ROM (only 64KB) but the same amount of RAM (340KB).
We could certainly provide more ROM by adding support for the external Quad SPI Flash (8MB).

The port is using a strategy similar to the shared stm32f429i-disc1/stm32f429i-disco. So not so much code is added, it just reuse the port from the stm32f746g-disco but declares a difference CPU model.

Since the board only has 64KB of flash, it's added to a lot of Makefile.ci, this change is in a separate because there are unrelated changes due to the use of the add_insufficient_memory_board.sh script.

Testing procedure

• Green Murdock
• Output of compile_and_test_for_board.py:

$ ./dist/tools/compile_and_test_for_board/compile_and_test_for_board.py . stm32f7508-dk --with-test-only --jobs=4
[...]
ERROR:stm32f7508-dk:Tests failed: 2
Failures during test:
- [tests/gnrc_rpl](tests/gnrc_rpl/test.failed)
- [tests/periph_timer_short_relative_set](tests/periph_timer_short_relative_set/test.failed)

I think the test failures are unrelated to this port.

Issues/PRs references

[benpicco]

Can we wire that up as a MTD device in SPI mode?

[aabadie]

Probably but I haven't tried (yet)