STM32 RTC : start LSI clock (for targets without LSE)

[jeromecoutant]

Description

RTC clock should be enabled as soon as possible after SW reset.

• for targets with LSE (most of targets with STM32), it is already working

• for targets using the internal clock (LSI), it was not the case, and all RTC registers were reset.

Test result

tests-mbed_hal-rtc_reset was FAIL for DISCO_F429ZI,
and is OK now

Pull request type

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

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3210
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8085/

Triggering tests

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

[mbed-ci]

Exporter Build : FAILURE

Build number : 2801
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8085/

[mbed-ci]

Test : SUCCESS

Build number : 3012
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8085/3012

[NirSonnenschein]

/morph export-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2806
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8085/