STM32F334xx wrong RAM size

[jeromecoutant]

Summary of changes

Issue detected in https://forums.mbed.com/t/sigsegv-porting-nucleo-f334r8-to-mbed-os-6/17053

STM32F334xx has 16K RAM, but not continuous.
So only 12K can be configured.

Note that issue comes from wrong information in Keil.STM32F3xx_DFP.2.2.2.pack
I reported the issue to ST pack experts.

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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[ciarmcom]

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

[0xc0170]

CI started

[mbed-ci]

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