powerpc/vdso: Refactor 32 bits and 64 bits pages setup

The setup of VDSO pages is identical for 32 bits VDSO and
64 bits VDSO.

Refactor that setup.

And use &vdsoXX_start which is synonym of vdsoXX_kbase.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/269ffb54c37fc1d46128f77d7a39f88ef4a9957d.1601197618.git.christophe.leroy@csgroup.eu