ARM: dts: mmp3-dell-ariel: Drop linux,usable-memory from /memory

Drop the linux,usable-memory properties; the schema is unhappy about
them.

They've been cargo-culted from Open Firmware and I don't know what
purpose they serve. Perhaps they are meant to provide the OFW runtime.
In that case it's still okay to drop them from here; OFW is welcome to add
it upon boot.

Signed-off-by: Lubomir Rintel <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]'
Signed-off-by: Arnd Bergmann <[e-mail]>

arch/arm/boot/dts/mmp3-dell-ariel.dts

···
 	};
 
 	memory@0 {
-		linux,usable-memory = <0x0 0x7f600000>;
 		available = <0x7f700000 0x7ff00000 0x00000000 0x7f600000>;
 		reg = <0x0 0x80000000>;
 		device_type = "memory";