powerpc/mm/hash: Update SDR1 size encoding as documented in ISA 3.0

ISA 3.0 document hash table size in bytes = 2^(HTABSIZE + 18)

No functionality change by this patch.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>