Add offset value, offset mode and cap value API's

[maustin-redant]

Have added options to:

1. Set offset mode in Offset Register for frequency adjustment
2. Set offset value in Offset Register as part of offset mode
3. Set CAP_SEL value in Control_1 Register

[jaredwolff]

@maustin-redant can you rebase? Just pushed some changes from @bandogora

[jaredwolff]

Dup of #6