stm32_generator: Add initial support for STM32MP1 processors

This modifies the stm32 symbol generator to support STM32MP1 processors. These processors support Linux and have fast parallel interfaces like DDR. They have large BGA packages and hundreds of pins. The existing generator is not really designed to deal with this many pins, so this also modifies the pin grouping to produce a smaller and better-organized symbol. I would also like to add support for creating multi-unit symbols, but I have deferred that to a subsequent series.

This series requires python 3.6 for f-string literals (though they are of course not terribly integral).