mcr20a use core_util_critical_section functions

[jarlamsa]

Description

Use core_util_critical_section functions for mcr20a when enabling and disabling interrupts.
This change is already done in the driver repository:
PelionIoT/mcr20a-rf-driver#23
Migrating the change now to mbed-os as well.

Pull request type

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

[jarlamsa]

@SeppoTakalo @artokin @kjbracey-arm please review

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3279
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8243/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2904
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8243/

[mbed-ci]

Test : SUCCESS

Build number : 3087
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8243/3087