Correct heap region calculation for Maxim targets

[ahmetalincak]

Summary of changes

I corrected heap region calculation for MAX32620, MAX32625 and MAX3263x.
Fixes #15237 and #14712.

Impact of changes

Migration actions required

Documentation

None

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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@harmut01 @0xc0170 @hugueskamba

[ciarmcom]

@ahmetalincak, thank you for your changes.
@hugueskamba @harmut01 @0xc0170 @ARMmbed/mbed-os-maintainers please review.

[hugueskamba]

LGTM

[0xc0170]

CI started

[mbed-ci]

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