MPS2 platform: Enable interrupt on rx for UART

[nismad01]

Description

Rx interrupt flag enabled, (in the same way as it is done in serial_api.c for TARGET_ARM_SSG/TARGET_IOTSS, TARGET_ARM_SSG/TARGET_BEETLE and other targets).
Tested with RawSerial class, which would not have any attached rx interrupt handler called without this fix.

Pull request type

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

[hug-dev]

Thanks for the fix! It is strange indeed that this variable was not set anywhere.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2680
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7622/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2317
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7622/

[mbed-ci]

Test : SUCCESS

Build number : 2418
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7622/2418

[0xc0170]

/morph mbed2-build