ARM: dts: mmp2-olpc-xo-1-75: Drop linux,usable-memory from /memory

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

mmp2-olpc-xo-1-75.dt.yaml: /: memory: False schema does not allow
{'linux,usable-memory': [[0, 528482304]],
'available': [[847872, 519245824, 4096, 782336]],
'reg': [[0, 536870912]], 'device_type': ['memory']}

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/mmp2-olpc-xo-1-75.dts

···
 	};
 
 	memory@0 {
-		linux,usable-memory = <0x0 0x1f800000>;
 		available = <0xcf000 0x1ef31000 0x1000 0xbf000>;
 		reg = <0x0 0x20000000>;
 		device_type = "memory";