soc: cva6: Add device tree node for RISC-V mtimer

The device tree entry for cva6 is currently missing a device tree node
for the mtime and mtimecmp registers in the core-local interrupt
controllers.
This causes the RISC-V machine timer driver not to be built, causing
build failures as the system clock is missing.
This commit rectifies this by adding the corresponding device tree
entry.

Signed-off-by: Eric Ackermann <eric.ackermann@cispa.de>