STM32: enable HSI/LSE clocks for LPUART

[bcostm]

Description

In LPUART baudrate function, check if the HSI/LSE clocks are enabled. If not then activate them. This allows for example to use the HSE as system clock and the HSI for the LPUART.

Should fix Issue #7469

CC @Eman869

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2588
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7498/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2230
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7498/

[mbed-ci]

Test : FAILURE

Build number : 2341
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7498/2341

[0xc0170]

/morph test

[mbed-ci]

Test : FAILURE

Build number : 2348
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7498/2348

[cmonr]

Hmmmmmm..... Suspicious.

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 2358
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7498/2358