[POWERPC] Use SLB size from the device tree

Currently we hardwire the number of SLBs to 64, but PAPR says we
should use the ibm,slb-size property to obtain the number of SLB
entries.  This uses this property instead of assuming 64.  If no
property is found, we assume 64 entries as before.

This soft patches the SLB handler, so it shouldn't change performance
at all.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>