STM32Gx: LSI clock selection when LSE is not available

[More-Wrong]

Summary of changes

Similar to #11525
When LSE was not available on these devices (STM32G4 and STM32G0), LSI would not be selectable due to an issue in HAL compatibility, resulting in a compile error.

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)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

This change was not, and is not part of any tests, however, compiling mbed for these devices with

"target.lse_available": 0

in the mbed_app.json file, without the changes, results in a compile error:

mbed-os/targets/TARGET_STM/mbed_overrides.c:154:39: error: 'RCC_RTCCLKSOURCE_NO_CLK' undeclared (first use in this function)
if (__HAL_RCC_GET_RTC_SOURCE() != RCC_RTCCLKSOURCE_NO_CLK) {

---

Reviewers

---

[ciarmcom]

@More-Wrong, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[mbed-ci]

Jenkins CI Test : ✔️ SUCCESS

Build Number: 1 | 🔒 Jenkins CI Job | 🌐 Logs & Artifacts

CLICK for Detailed Summary

jobs	Status
jenkins-ci/mbed-os-ci_unittests	✔️
jenkins-ci/mbed-os-ci_build-ARM	✔️
jenkins-ci/mbed-os-ci_build-GCC_ARM	✔️
jenkins-ci/mbed-os-ci_greentea-test	✔️
jenkins-ci/mbed-os-ci_dynamic-memory-usage	✔️
jenkins-ci/mbed-os-ci_cloud-client-pytest	✔️