arm: dts: am33xx.dtsi: Add node name to rtc device node

Making it possible to reference and therefor change (disable) this
device node from other dts file which import this dtsi file.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Lukas Stockmann <lukas.stockmann@siemens.com>
Cc: Benoit Cousson <bcousson@baylibre.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>