PSA: Fix compile error with NV seed

[ccli8]

Summary of changes

This PR tries to fix compile error which can reproduce when PSA V7M and MBEDTLS_ENTROPY_NV_SEED are both enabled. To reproduce the error, in mbed-os-example-blinky (tag mbed-os-6.12.0), add mbed_app.json with:

{
    "macros": [
        "MBEDTLS_ENTROPY_NV_SEED"
    ],
    "target_overrides": {
        "*": {
            "target.features_add": ["EXPERIMENTAL_API"]
        }
    }
}

Then build for K64 running:

mbed compile -m K64F -t ARM

Then meet compile error:

[Error] pal_mbed_os_intf.cpp@112,18: use of undeclared identifier 'MBEDTLS_ENTROPY_BLOCK_SIZE'

---

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)

---

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

---

[ciarmcom]

@ccli8, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[Patater]

CI started

[mbed-ci]

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jenkins-ci/mbed-os-ci_build-cloud-example-ARM	✔️
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jenkins-ci/mbed-os-ci_build-example-GCC_ARM	✔️
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jenkins-ci/mbed-os-ci_build-example-ARM	✔️
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