dm_rtc: bootcount rtc driver

[thom-nic]

dts example:

	chosen {
		// see: u-boot/drivers/bootcount/bootcount-uclass.c
		u-boot,bootcount-device = &bootcount_rv3028;
	};

	/* Phycore contains an RV-3028-C7 RTC with user EEPROM:
	   https://www.microcrystal.com/fileadmin/Media/Products/RTC/App.Manual/RV-3028-C7_App-Manual.pdf
	   ensure the following config are enabled:
	   - CONFIG_DM_BOOTCOUNT_RTC
	   - CONFIG_RV3028
	  NOTE: u-boot/drivers/rtc/rv3028.c does not appear to support EEPROM access so we can only use
	  the two "User RAM" registers at 0x1F-0x20
	*/
	bootcount_rv3028: bc_rv3028 {
		// see: u-boot/drivers/bootcount/rtc.c
		compatible = "u-boot,bootcount-rtc";
		rtc = <&i2c_som_rtc>;
		offset = <0x1F>; // registers 0x1F-0x20 are "User RAM"
		/* NOTE: the linux rtc-rv3028 driver exposes *two* nvmem devices, one for the
		   user RAM registers and one for the EEPROM.
		   See: https://github.com/torvalds/linux/blob/v6.12/drivers/rtc/rtc-rv3028.c#L920-L927
		   These hints help the bootcount userspace program choose the correct nvmem device:
		*/
		linux,nvmem-type = "Battery backed";
		linux,nvmem-offset = <0x00>;
	};

Any rtc driver that exposes nvmem should work but the RV3028 is an example.

• fixes support rtc nvmem #20