spapr: Add "slb-size" property to CPU device tree nodes

spapr: Add "slb-size" property to CPU device tree nodes

According to a commit message in the Linux kernel (see here
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=b60c31d85a2a
for example), the name of the property that carries the information
about the number of SLB entries should be called "slb-size", and
not "ibm,slb-size". The Linux kernel can deal with both names, but
to be on the safe side we should support the official name, too.

[Now that LoPAPR is public, the relevant requirement can be found in
section C.6.1.8 --dwg]

Signed-off-by: Thomas Huth <[email protected]>
Signed-off-by: David Gibson <[email protected]>