riscv: use relative path to include contrib's data

riscv: use relative path to include contrib's data

Doxygen cannot resolve the path of the files in folder contrib.
Use a path relative to current folder, as done in other files.

Change-Id: If39b416ed422b4854dd108777fa32dd4c809450a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6635
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>