tegra: Improve the detection of the secondary function (RTC) of the MAX77620.

Use the new ofw_iicbus_set_devinfo() method to implant an OFW compatibility
string for a manually created RTC sub-device.

MFC after: 4 weeks
Reported by: archimedes.gaviola_at_gmail.com

		bscott_at_bunyatech.com.au