Correct PB_6/PB_7 Serial AF mapping

[amitchone]

Description (required)

Pins PB_6 and PB_7 had incorrect PinMap_UART_TX/RX definitions. Attempting to initialise a Serial object using these pins ultimately led to vastly increased power consumption (by ~40mA with target default clock settings) and a UART that didn't work.

This pull request stems from this issue.

Summary of change (What the change is for and why)

To correct pins PB_6 and PB_7 Serial PinMap.

Documentation (Details of any document updates required)

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Pull request type (required)

[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 (required)

[x] 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

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Reviewers (optional)

@kjbracey-arm

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[jeromecoutant]

Code correct in TARGET_DISCO_L072CZ_LRWAN1

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts