svdpatch: create fields element if register has no fields

svdpatch: Fix _replace_enum when used on derived fields

Make sure the enum is replaced properly in the case of derived
enumerated values.

Signed-off-by: Rahix <rahix@rahix.de>

svdpatch: Fix peripheral modifications

svdpatch: Add _write_constraint modifier

"_write_constraint" allows to change the write-constraint
to one of:

- "enum": Allow only enumerated values (useEnumeratedValues)
- "none": Completely remove the <writeConstraint> tag
- [min, max]: Allow only values in a certain range (this can
  be used to make the bits() method safe)

Fixes #8

Signed-off-by: Rahix <rahix@rahix.de>

svdpatch: Fix field modifications

Signed-off-by: Rahix <rahix@rahix.de>

Allow overwriting enumeratedValues and add exemplary patch

Integrate patching in the build process

Integrate svdpatch.py of stm32-rs