powerpc/nohash: Replace #ifdef CONFIG_44x by IS_ENABLED(CONFIG_44x) in pgtable.h

No need of a #ifdef, use IS_ENABLED(CONFIG_44x)

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/7c7d97322a6a05a9842b1e8c4b41265916f542ca.1695659959.git.christophe.leroy@csgroup.eu