dts: rename 'vexriscv,intc0' compatible to 'vexriscv-intc0'

As far as I can tell, 'vexriscv' does not name a vendor:

https://github.com/SpinalHDL/VexRiscv



It also doesn't seem entrenched enough to merit a special case
exception to the de factor rule 'the "vnd,foo" namespace is for
vendors'. This is open to debate and we can revise as needed in the
future, but for now let's just rename the compatible to avoid
triggering warnings/errors about unknown vendors.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>