Support "derivedFrom" attribute

[konovod]

I've found this attributes in two places:

1. peripherals

<peripheral derivedFrom="GPIOA">
  <name>GPIOB</name>
  <baseAddress>0x40010C00</baseAddress>
</peripheral>

Not sure how rust process it, but perhaps we should simply copy content of another peripheral and then apply changes.
Better way would be to define generic module and then extend it, but it seems more difficult (even deriving name GPIO by finding common substring from GPIOA and GPIOB looks hacky).

2. <enumeratedValues derivedFrom="CTB2"/>
   In this context I think it is better to simply reuse already generated enum.

[RX14]

I think I will probably apply the duplication method. Not sure how enum inheritance works in Crystal though, perhaps it's better to always use a new enum. Though, I would like to collect up duplicate enums somewhere in a post-processing pass eventually.

[RX14]

I've been not working on embedded crystal the past few weeks due to working on a new electronics project instead, but after that's done (soon) I should have a new reason to work on embedded crystal again, and more peripherals to test.