KL82Z: Update the clock selection method for LPUART module

[mmahadevan108]

Fixes UART issues seen on the KL82Z FRDM board.

[mmahadevan108]

cc @thinkberg Please take a look at the change for the USENSE board.

[mmahadevan108]

cc @0xc0170 @maclobdell

[thinkberg]

looks okay to me

[0xc0170]

/morph build

[mbed-ci]

Build : FAILURE

Build number : 1598
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6467/

[cmonr]

ARM network license issue.

Rebuilding:
/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1602
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6467/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1234
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6467/

[mbed-ci]

Test : SUCCESS

Build number : 1388
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6467/1388