STM32: define RCC_LSE_BYPASS to allow using PC_15 as gpio

[facchinm]

Summary of changes

This is the first part of a PR series to support Arduino Portenta H7 .
The patchset has been split into independent PRs for an easier review.
#13816

This PR content:

1. Add a global mbed configuration define to use STM32 LSE_BYPASS option releasing PC_15

/cc @pennam @pnndra

Impact of changes

All STM32 targets with LSE available (MBED_CONF_TARGET_LSE_AVAILABLE)

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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[ciarmcom]

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

[0xc0170]

CI started

Set to patch version (target update only)

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[jeromecoutant]

I suppose this "lse-bypass" config is set for Portenta in targets.json

[jeromecoutant]

STM32 information :-)

• When the LSE oscillator is switched ON (RCC_LSE_ON) the oscillator takes control of its associated pins
  (PC_14 and PC_15, named as RCC_OSC32_IN and RCC_OSC32_OUT in PinNames.h file),
  then you can't use these pins as GPIO.
• When the oscillator is configured in a user external clock mode (RCC_LSE_BYPASS),
  only the OSC32_IN (PC_14) pin is reserved for clock input
  and the OSC32_OUT (PC_15) pin can be used as normal GPIO.

Same feature for HSE.

[facchinm]

@jeromecoutant yup! 51cff23

[0xc0170]

Travis run (due to the travis org change, we got 2 statuses). I'll merge.