Update rtc_api.c

[star297]

Removed redundant RTC->CR define code + added RTC compensation code to allow calibration of the RTC.

Tom Russell found the RTC->CR settings in the rtc_api.c file overrode the OSC0->CR setting in system_MKL05Z4.c.
Effectively removing the internal loading capacitors for the external 32KHz crystal when the RTC is initialised.
If the external crystal 32Khz clock source is selected (default on the KL05), it is not necessary to call the RTC->CR functions. All other FRDM KLxx platforms RTC clock source is derived from the interface MCU.

Added RTC compensation function to allow calibration of the RTC.
This needs to be set in the rtc_init area, according to the manual, once the locks are enabled they can only be disabled with a POR or software reset making it difficult, probably impossible to place it in the user program code.

[0xc0170]

cc @Sissors

[Sissors]

Can't we remove the OSC->CR overwriting in the rtc_api file, so the clock setup is done in the system file?

[star297]

That already happens in the clock set up Erik, hence removing the overwrite in the rtc_api.c
I can't think of any reason why it would be needed in the rtc_api.c it could only be relevant when the clock set up is using 32KHz external crystal. Other than using the internal slow 32KHz IRC to clock the RTC, but who is going to use that, too inaccurate.

The added compensation code does what is says on the box, but couldn't get this working in the main user code area which would be preferable to save modifying Mbed-dev files. According to the manual I believe the register locks are fixed after clock set up has initialised only to be released on POR or soft reset.
But I may have missed something.

[Sissors]

My bad, I didn't properly read your code :(. This indeed seems good to me.