dts: arm: st: n6: Use dedicated dts node for NPU cache (aka CACHEAXI)

The new node is called "npu_cache".

This way a possibility is offered to choose - thru an overlay - if to
enable the NPU cache or not.
This new node has a dependency with node "npu", so the NPU cache's
status is taken into account only in case node "npu" has status "okay".

Default status value of "npu_cache" is "okay"
(i.e. enable the NPU cache).

Signed-off-by: Wolfgang Betz <wolfgang.betz@st.com>