Enable DeviceKey class reference for Doxygen

[AnotherButler]

Description

Add conditional, so Doxygen renders properly.

Pull request type

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

CC @yossi2le

[AnotherButler]

@cmonr @adbridge @ARMmbed/mbed-os-maintainers If possible, could this please go into 5.9? If not, could this please go into 5.9.1?

[cmonr]

Good with me.

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2144
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7012/

Triggering tests

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

[mbed-ci]

Test : SUCCESS

Build number : 1938
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7012/1938

[mbed-ci]

Exporter Build : FAILURE

Build number : 1772
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7012/

[0xc0170]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1776
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7012/