dts: Move nrf_common.dtsi to the common directory

... so that it can be included by ARM and RISC-V cores. For the same
reason, SysTick can no longer be disabled in this common file.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>