Cellular: Enable ePCO IE for BG96 driver

[AriParkkila]

Description (required)

As per 3GPP TS 24.301:

If the UE supports NB-S1 mode, Non-IP PDN type, or N1 mode,
then the UE shall support the extended protocol configuration options IE.
...
If the UE supports the extended protocol configuration options IE,
then the UE shall set the ePCO bit to extended protocol configuration options supported
in the UE network capability IE of the ATTACH REQUEST message.

Summary of change (What the change is for and why)

Documentation (Details of any document updates required)

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Pull request type (required)

[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 (required)

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers (optional)

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Release Notes (required for feature/major PRs)

Summary of changes

Impact of changes

Migration actions required

[ciarmcom]

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

[0xc0170]

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[mbed-ci]

Test run: SUCCESS

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