[nRF52] UART can support 2 stop bits configuration

[facchinm]

Description of defect

This assert

mbed-os/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/serial_api.c

Line 1095 in f38aa59

MBED_ASSERT(stop_bits == 1);

is wrong, at least for nRF52.
The chip supports 1 or 2 stop bits as per datasheet

Cross referencing with arduino/ArduinoCore-mbed#60

Target(s) affected by this defect ?

Nordic nRF52

Toolchain(s) (name and version) displaying this defect ?

Any

What version of Mbed-os are you using (tag or sha) ?

f38aa59

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

ArduinoCore-mbed, mbed-cli

How is this defect reproduced ?

Configure the UART for 2 stop bits, the assert will fire.

[ciarmcom]

@facchinm thank you for raising this issue.Please take a look at the following comments:

We cannot automatically identify a release based on the version of Mbed OS that you have provided.
Please provide either a single valid sha of the form #abcde12 or #3b8265d70af32261311a06e423ca33434d8d80de
or a single valid release tag of the form mbed-os-x.y.z .
E.g. 'mbed-os-99.99.99' signifies master branch and is not a unique single snapshot.
NOTE: If there are fields which are not applicable then please just add 'n/a' or 'None'.This indicates to us that at least all the fields have been considered.
Please update the issue header with the missing information, the issue will not be mirroredto our internal defect tracking system or investigated until this has been fully resolved.

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers.
Internal Jira reference: https://jira.arm.com/browse/IOTOSM-2324