Add byte-swapping versions of __raw_writeq() and __raw_rm_writeq().

This allows us to avoid sparse warnings caused by passing __be64 to
__raw_writeq(), which takes unsigned long:

  arch/powerpc/platforms/powernv/pci-ioda.c:1981:38:
  warning: incorrect type in argument 1 (different base types)
      expected unsigned long [unsigned] v
      got restricted __be64 [usertype] <noident>

It's also generally preferable to use a byte-swapping accessor rather
than doing it by hand in the code, which is more bug prone.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>