Rewrite peripherals declaration and definition logic
[STM32F0] [GPIO] Fix length of MODER and PUPDR masks
[CORTEX-M0] Add common memory map
Define _RV - reserved register type as readonly

Reserved registers must either be kept at reset value in which case
making them readonly makes sense, or writing to them has no effect
in which case there is no point writing to them and may as well
be read only.
[CORTEX-M0] Add common peripherals definitions
small (mainly stylistic) changes before merging