arm64: dts: stratix10/agilex/n5x: fix dtbs_check warning for memory node

Although, we expect the bootloader to full memory details but passing empty
values can give warning, so add a default value.

memory: False schema does not allow {'device_type': ['memory'], 'reg': [[0, 0, 0, 0]]}

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>