docs: fix CNTRL_2 DATA_FORMAT field

[spectrum70]

Noted that in my application, ad3552r-axi, i need to set this bit (4) to 0 to have unsigned 16bit samples generated.

PR Description

Fix DAC IP documentation.

PR Type

• Bug fix (change that fixes an issue)
• New feature (change that adds new functionality)
• Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• I have followed the code style guidelines
• I have performed a self-review of changes
• I have compiled all hdl projects and libraries affected by this PR
• I have tested in hardware affected projects, at least on relevant boards
• I have commented my code, at least hard-to-understand parts
• I have signed off all commits from this PR
• I have updated the documentation (wiki pages, ReadMe files, Copyright etc)
• I have not introduced new Warnings/Critical Warnings on compilation
• I have added new hdl testbenches or updated existing ones

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[sarpadi]

Tagging @caosjr

[PopPaul2021]

Hello @spectrum70,

The AD3552R requires the data to be in offset binary format on the QSPI interface. We can verify this by replacing the binary code with 0 or 2^16-1 in the Transfer function formula (datasheet page 27). Therefore, we need to set the bit to one to convert the data from the DMA or the ADC from 2’s complement to offset binary.