STM32F429ZI add SERIAL_ASYNCH

[pilotak]

Related to #6080

[mbed-ci]

--none--

[0xc0170]

@pilotak did you run any tests for this change?

[ohagendorf]

Exactly the same MCU in another target (DISCO_F429ZI) had the SERIAL_ASYNCH flag already set.

[pilotak]

compiles fine, i don't know what other tests you need

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1146
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6088/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 821
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6088/

[mbed-ci]

Test : SUCCESS

Build number : 952
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6088/952

[cmonr]

@pilotak For future reference, the test results that we're looking for come from the command mbed test

I also ran a local instance with this PR against GCC, and results were good.

[pilotak]

@cmonr Ok that's fine will test it next time