[svd parser] CPU block incorrectly requires mpuPresent and fpuPresent elements

[xoriath]

I noticed a discrepancy between the documentation and the actual SVD schema for these 2 elements; see Open-CMSIS-Pack/svd-spec#32

I've send a PR to update the documentation to conform to the SVD schema. This change also needs a loosening in the svd-parser here... Default to false if these elements are not present.