riscv: dts: thead: Add missing GPIO clock-names

The gpio-dwapb looks for clock named "bus" so add clock-names property
for the gpio controller nodes.

Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
[dfustini: add two more lines to the commit message]
Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>