Rewrite `stm32f4` setup with svd generated registers#18

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mitchmindtree wants to merge 2 commits intostm32-rs:masterfrom

mitchmindtree:stm32f4_setup

Commits on Jul 19, 2020

Rewrite `stm32f4` setup with svd generated registers
mitchmindtree
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Commits on Jul 28, 2020

Use modify rather than write in stm32f407 setup
mitchmindtree
committed