powerpc: Add __raw_rm_writeq() function

Move __raw_rm_writeq() from platforms/powernv/pci-ioda.c to
include/asm/io.h so that it can be used by other code.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>