STM32L4: Use HSI for LPUART

[jeromecoutant]

Summary of changes

Using low power uart instance has some constraint about clock source selection.

See:
https://github.com/ARMmbed/mbed-os/blob/master/targets/TARGET_STM/serial_api.c#L368

Default choice is LSE and PCLK1.

Issue was seen with DISCO_L475VG_IOT01A:

mbed test -t ARM -m DISCO_L475VG_IOT01A -v --app-config tests/configs/fpga.json -n hal-tests-tests-mbed_hal_fpga_ci_test_shield-uart -DFPGA_FORCE_ALL_PORTS

Tests with LPUART and 9600 baud rate are OK, not with 19200 (PCLK1 is 80MHz).

Impact of changes

Impact all STM32L4x5 and STM32L4x6 targets
For these targets, LPUART clack source is HSI (16 MHz)

Migration actions required

Documentation

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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)

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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

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Reviewers

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