powerpc/mm: Leave a gap between early allocated IO areas

Vmalloc system leaves a gap between allocated areas. It helps catching
overflows.

Do the same for IO areas which are allocated with early_ioremap_range()
until slab_is_available().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/c433e358190fb5d47650463ea1ab755fc7b73e6e.1618828806.git.christophe.leroy@csgroup.eu