[Feature] Add switch to inhibit generating reserved fields

[ThyMYthOS]

svd2ada "invents" reserved fields when it detects a gap in the register space. This behaviour should at least be configurable.

[damaki]

Omitting the reserved fields would not be safe as it would leave gaps in the register record representation, which could result in junk data (whatever happens to be in those bits of the register at the time) being written to the reserved fields when the register is assigned. Hardware typically expects a specific value to be used for reserved bits (usually 0, sometimes 1), so writing an indeterminate value to the reserved bits could cause unexpected behaviour.

[ThyMYthOS]

Sure that's why it should be a switch that is off by default.