ARM: dts: am335x-phycore-som: Add alias for TPS65910 RTC

Without an alias for the TPS65910 RTC, it snatches the rtc0 device in
advance to the I2C RTC assigned to that alias.

Signed-off-by: Steffen Hemer <s.hemer@phytec.de>
Message-Id: <20230214132302.39087-1-s.hemer@phytec.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>