powerpc: Drop page_is_ram() and walk_system_ram_range()

Since commit c40dd2f7 ("powerpc: Add System RAM to /proc/iomem")
it is possible to use the generic walk_system_ram_range() and
the generic page_is_ram().

To enable the use of walk_system_ram_range() by the IBM EHEA ethernet
driver, we still need an export of the generic function.

As powerpc was the only user of CONFIG_ARCH_HAS_WALK_MEMORY, the
ifdef around the generic walk_system_ram_range() has become useless
and can be dropped.

Fixes: c40dd2f7 ("powerpc: Add System RAM to /proc/iomem")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[mpe: Keep the EXPORT_SYMBOL_GPL in powerpc code]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>