M2351: Fix some minor issues

[ccli8]

Summary of changes

This PR tries to fix the following issues on M2351. Secure image/lib is not influenced by this change, so no need to rebuild it.

1. Fix NSC_Init(...).
2. Fix interrupt vector with BSP update.
3. Fix RTC comment

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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

[ciarmcom]

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

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CI started

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CI restarted (internal error)

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 2
Build artifacts