soc: riscv: gd32vf103: use nuclei,systimer compatible

After some analysis I found out that there's no machine timer provided
by the "riscv" vendor. There are some specs for the mtime/mtimecmp
registers (this is why we can have a single driver), but the actual
register layout or implementations differ amongst vendors. GD32 uses the
Nuclei implementation, named "system timer" in their documentation. This
patch aligns with vendor specs.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>