bootcount: add a DM RTC backing store for bootcount

bootcount: add a DM RTC backing store for bootcount

This implements a driver using a RTC-based backing store for the DM
bootcount implementation.  The node configuring this feature will be
compatible with 'u-boot,bootcount-rtc' and the underlying RTC device
shall be reference through the property 'rtc'. An offset into the RTC
device's register space can be provided through the 'offset' property.

Tested on a RK3399-Q7 on a Flamingo carrier board using the SRAM area
of the carrier board's RV3029 RTC.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>