dts: riscv: virt: use sifive,clint0

Use the sifive,clint0 compatible instead of "riscv,clint0" and remove
interrupt controller fields (clint compatible is used for its timer
registers). Refer to the Linux or previous commits for more context.

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