STM32L1: add support of MCU_STM32L151xB

[jeromecoutant]

Summary of changes

Add definition of MCU_STM32L151xB in the json file
in order to ease custom boards like RAK811

Bonus: add MCU_STM32L151xBA which is the enhanced version with:

• more RAM
• the correct RTC IP version needed for MBED LPTICKER support

Impact of changes

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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[jeromecoutant]

Verified with
ARMmbed/stm32customtargets@9452c38

[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

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