arm64: dts: ti: k3-j721e-main: Correct main NAVSS representation

NAVSS is a subsystem containing different IPs, it is not really a bus.
Change the compatible from "simple-bus" to "simple-mfd" to reflect that.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>