powerpc/ioremap: warn on early use of ioremap()

Powerpc now has EARLY_IOREMAP.

Next step is to convert all early users of ioremap() to
early_ioremap().

Add a warning to help locate those users.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b4f03a68ee8e68773c8973d74ec35f9c82c72871.1568295907.git.christophe.leroy@c-s.fr