powerpc/prom: Simplify the logic to fetch SLB size

The code to fetch the SLB size from the device tree wants to first look
for "slb-size" and then if that's not found "ibm,slb-size".

We can simplify the code by looking for the properties and then if we
find one of them we set mmu_slb_size.

We also change the function name from check_cpu_slb_size() to
init_mmu_slb_size() as the function doesn't check anything, it only
initialises mmu_slb_size.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
[mpe: Rewrite change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>