Cellular: Refactor socket_stack_init() from generic to modem specific

[AriParkkila]

Summary of changes

AT_CellularStack::socket_stack_init is not needed in generic AT drivers, so refactor it to modem specific driver classes.

Impact of changes

Migration actions required

Documentation

---

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

---

Reviewers

---

[ciarmcom]

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

[adbridge]

Ci started

[mbed-ci]

Test run: SUCCESS

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

[mergify]

This PR does not contain release version label after merging.