Cellular: Add getters for device and serial to EasyCellularConnection

[mirelachirica]

Description

Addresses need to have direct access from application to power interface:
#7470 (comment)

Pull request type

[ ] Fix
[ ] Refactor
[ ] Target update
[x ] Feature
[ ] Breaking change

[0xc0170]

@wajahat-ublox Please review

[wajahat-abbas]

Looks good as this will resolve the problem of not having access to AT_CellularPower::opt_power_save_mode

Sorry for late response, i was on vacations.

[AnttiKauppila]

@0xc0170 Can you trigger this build? We would like to make it in 5.10.0-rc1

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2918
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7795/

Triggering tests

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

[cmonr]

Something hung up the export test. Restarting.

/morph export-build

[mbed-ci]

Test : FAILURE

Build number : 2667
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7795/2667

[cmonr]

Will restart once CI load is lower...

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2541
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7795/

[0xc0170]

/morph test

[mbed-ci]

Test : FAILURE

Build number : 2684
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7795/2684

[0xc0170]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 2694
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7795/2694

[NirSonnenschein]

CI seems to have passed, but missing a review @0xc0170