hdl: driver: add subscripted VCTCXO read/write support

The VCTCXO DAC operations now have the same functionality as GPIO reads/writes.
The Nios core stores the least significant byte in a temporary variable and only
performs a write to the DAC when the most sigificant byte of the 16bit DAC value
is written to it by the host.

This commit does not address host endianness.