MLBEDSW-9267: Fix incorrect Shape hash affecting tensor cache

The tensor cache was seen to be holding duplicate encoded tensors
all with the same parameters but a different key hash. This turned
out to be a fault in the Shape hash (which always combined 4 fields
regardless of actual shape rank) generating unstable values for
3-axis architecture ublocks.

 - Updated Shape hash to incorporate only all valid axes.

Signed-off-by: Philip Hall <philip.hall@arm.com>
Change-Id: Iae11eeec0eb343caf77e8149b968ba53ba7d8afc