powerpc: Get the number of SLBs from "slb-size" property
The PAPR says that the property for specifying the number of SLBs should
be called "slb-size". We currently only look for "ibm,slb-size" because
this is what firmware actually presents.
This patch makes us look for the "slb-size" property as well and in
preference to the "ibm,slb-size". This should future proof us if
firmware changes to match PAPR.
Signed-off-by: Michael Neuling <mikey@neuling.org> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>