ARM: dts: AM4372: Add the am4372-rtc compatible string

am4372-rtc string was already part of dts, introduced to identify
the rtc specific to am4372 family of SoCs. It was removed in one of the
previous patches. Adding back the same with appropriate documentation.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>