Summary of changes

I noticed that Mbed OS does not currently build for STM32U0, because updating a section of code was missed during the QSPI DMA PR a while back. This happened because we forgot to create a CI build job for this target (oops!). This PR fixes the build issue and adds the missing build job.

Impact of changes

Migration actions required

Documentation

None

Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

Test results

[] No Tests required for this change (E.g docs only update)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR