Add support for GENERIC_EARLY_IOREMAP.

Let's define 16 slots of 256Kbytes each for early ioremap.
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/412c7eaa6a373d8f82a3c3ee01e6a65a1a6589de.1568295907.git.christophe.leroy@c-s.fr