Update SVD2Swift to generate RegisterArrays

[rauhul]

Updates SVD2Swift to automatically generate RegisterArrays for
dimensioned SVD instances (excluding fields) instead of N instances
being generated as peers suffixed by an index.

Introduces RegisterProtocol to standardize the API for register types
needed for RegisterArray to work with both @RegisterBlocks and
Registers.

Updates the XML decoding library and macro implementation to handle
decoding optional @XMLInlineElement properties and Updates the SVD
model structures, such as SVDCluster, to make dimensionElement
optional with non-optional child properties.

Consolidates the @XMLAttribute and @XMLInlineElement macro
implementations.