Fix issue #5119, changed pwmout_api.

[MateuszMaz]

Description

K64F was not able to generate pwm with 100 ms long period. This PR fix the problem, by computing and setting the optimal prescaler value while setting the pwm period. This way we also don't lose accuracy for short periods, but it is even enhanced a little.

Pull request type

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

[fkjagodzinski]

CC @0xc0170 @jamesbeyond @mprse @maciejbocianski

[0xc0170]

this is target code but should be same as the rest of the code here, see https://os.mbed.com/docs/latest/reference/style.html

spaces around operators and while (), etc. Please fix

[0xc0170]

pwm_clock_mhz * (float)us spaces around operators, same for line 135

[MateuszMaz]

@0xc0170 formatting updated

[mmahadevan108]

Thanks for the patch

[cmonr]

Interesting. I'm good with the change, since imo most common applications set a fixed frequency instead of duty cycle.

@ARMmbed/mbed-os-hal Care to take a quick look?

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2799
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7716/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2429
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7716/

[cmonr]

/morph test

[cmonr]

/morph mbed2-build

[mbed-ci]

Test : SUCCESS

Build number : 2531
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7716/2531