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 <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20210121034130.1381872-6-lkundrak@v3.sk'
Signed-off-by: Arnd Bergmann <arnd@arndb.de>