Add low power timer fallback for platforms without RTC

[bulislaw]

Description

Low power timer will be used as RTC for platforms that don't have HW RTC
capabilities. It will help maintain compatibility between targets and will ensure backward compatibility for targets that will lose RTC support when new HAL RTC API is merged to master.

Pull request type

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

[cmonr]

Is there any particular reason this and line 30 aren't merged?

[bulislaw]

Fixed.

[bulislaw]

Can we start the testing please.

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2062
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6878/

Triggering tests

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

[mbed-ci]

Test : FAILURE

Build number : 1873
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6878/1873

[bulislaw]

This PR doesn't touch anything around F401 or lptimer.
/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2069
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6878/

Triggering tests

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

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1709
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6878/

[mbed-ci]

Test : SUCCESS

Build number : 1879
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6878/1879

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1715
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6878/

[adbridge]

@cmonr Are you happy with this?

Testing has passed but this still needs a review from one of the following:
@c1728p9 @SenRamakri Could one of you please review this ASAP.

[c1728p9]

Do devices without an RTC typically have a low power timer? If not, the low power timer may need to fall back to the microsecond timer.

[SenRamakri]

I see that _rtc_lp_base is getting written in the write function below, how is it used?

[bulislaw]

_rtc_lp_base is used as a start time and then we add the actual timer value to it as timer starts from 0.

[SenRamakri]

Overall it looks good to me. I have one question which I have commented.

[adbridge]

@bulislaw Could you please answer the questions asked ? Though this has not been blocked on them.

[bulislaw]

Done