dts: riscv: pulpino: Fix warning for itim unit address

dtc produces the following warning:

Warning (simple_bus_reg): /soc/itim@8000000: simple-bus unit address
format error, expected "0"

We had the wrong unit address in the dtsi file, should be 0.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>