arch: arm: minor style fixes in dts binding .yaml files

This commit updates the title and the description of the
ARMv6-M, ARMv7-M NVIC dts bindings, so that both the title
and the description reference the respective ARM architecture
and not the Cortex M cores.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>