M467: Replace PlatformMutex with rtos::Mutex

[ccli8]

Summary of changes

This PR replaces PlatformMutex with rtos::Mutex for supporting both mbed-baremetal and mbed-os builds.

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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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[ccli8]

Hi, this is my first time to submit to mbed-ce. Would you take a look at this?

[JohnK1987]

Hi, this is my first time to submit to mbed-ce. Would you take a look at this?

Hello,

thank you for the PR and welcome to the party.

BR, Jan