Document the PowerPC specific 'sleep' property as a schema. It is
currently only documented in booting-without-of.rst which is getting
removed.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201008142420.2083861-1-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>