powerpc/io: Expect immutable pointer in virt_to_phys() prototype

virt_to_phys() doesn't need the address pointer to be mutable.

At the same time allowing it to be mutable leads to the following build
warning for constant pointers:

  warning: passing argument 1 of ‘virt_to_phys’ discards ‘const’ qualifier from pointer target type

Signed-off-by: Stanislav Kinsburskii <stanislav.kinsburskii@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/168155747391.13678.10634415747614468991.stgit@skinsburskii.localdomain