M64's are the configurable 64-bit windows that cover the 64-bit MMIO
space. We used to hard code 16 windows. Newer chips might have a
variable number and might need to reserve some as well (for example
on PHB4/POWER9, M32 and M64 are actually unified and we use M64#0
to map the 32-bit space).

So newer OPALs will provide a property we can use to know what range
of windows is available. The property is named so that it can
eventually support multiple ranges but we only use the first one for
now.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>