Arm backend: Update operator support for TOSA-1.0+INT+u55 (#10849)

### Summary
Make is_U55_subset part of the base TOSA specification class as the
subset is not tied to a specific specification. Using the
is_U55_subset attribute the supported checks for U55 subset are
updated to catch TOSA 1.0 as well.

### Test plan
Tested on internal and external CI.


Signed-off-by: Per Åstrand <per.astrand@arm.com>