powerpc: Cast away __iomem in low level IO routines

Sparse reports dereferencing an __iomem pointer. These routines
are clearly low level handlers for IO memory, so force cast away
the __iomem annotation to tell sparse the dereferences are safe.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231011053711.93427-11-bgray@linux.ibm.com