[regtool] remove unused empty line from `-d` output

[m-temp]

Description

Problem

If regtool is used to generate the register-documentation, for example ./util/regtool.py -d hw/ip/ascon/data/ascon.hjson > hw/ip/ascon/doc/registers.md The markdown file ends with two empty lines. However, this is triggers the Lint in the CI:

Would have fixed 1 files.
Fixing file: "hw/ip/ascon/doc/registers.md"
##vso[task.logissue type=error]Whitespace check failed. Please run util/fix_trailing_whitespace.py on the above files.

Running this command then removes one of the empty lines.

Solution

Regtool shouldn't add this duplicate empty line in the first place