Storage features: fix static pin-map for ARMC5

[mprse]

Summary of changes

constexpr specified can not be used with ARMC5 compiler and is causing build failures.
Switching constexpr to const should not consume extra ROM (in case of ARMC6) and should be ok to ARMC5.

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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[kjbracey]

Negative side-effect for ARMC5 is that static_spi_pinmap and the HAL SPI pinmap table and computation code will be pulled in even if not referenced.

In theory, removing the constexpr would permit the same effect to happen on other compilers with badly-written HALs - removing the constexpr gets rid of the check that get_spi_pinmap is compile-time, not runtime.

But I guess we'd spot that in code size if it occurred. (Or maybe we have Greentea tests that check the get_pinmaps really are constexpr?)

[adbridge]

CI started

[mbed-ci]

Test run: SUCCESS

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

[0xc0170]

fixes #12103 ?

[mprse]

fixes #12103 ?

I believe it does.