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>