Added SerialWireOutput to mbed namespace

[AGlass0fMilk]

Description

Added the SerialWireOutput driver to the mbed namespace

See issue #6421

See why this is a good idea in #5679

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[marcuschangarm]

Thank you for fixing this!

[cmonr]

Cool. Looks reasonable.
@AGlass0fMilk Thanks for the issue references.

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1527
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6427/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1169
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6427/

[mbed-ci]

Test : SUCCESS

Build number : 1309
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6427/1309

[kjbracey]

This would be a breaking change for anyone using it who hadn't included "mbed.h" to get their compulsory using namespace mbed. I'm fine with that, but think it makes it a 5.9 thing.