[nrf fromlist] boards: nrf54h20dk: Drop shared_ram20_region's compatible

Upstream PR: zephyrproject-rtos/zephyr#72273

Access to this region should no longer be requested via UICR, because it
will be statically allocated by secure domain firmware.

Signed-off-by: Grzegorz Swiderski <[email protected]>
(cherry picked from commit d95aa856e98b6561d534cbe36ffc6e87de3deab2)

Author: 57300

boards/arm/nrf54h20dk_nrf54h20/nrf54h20dk_nrf54h20-memory_map.dtsi

@@ -70,11 +70,7 @@
 		};
 
 		shared_ram20_region: memory@2f88f000 {
-			compatible = "nordic,owned-memory";
 			reg = <0x2f88f000 DT_SIZE_K(4)>;
-			status = "okay";
-			perm-read;
-			perm-write;
 			#address-cells = <1>;
 			#size-cells = <1>;
 			ranges = <0x0 0x2f88f000 0x1000>;