alpha: add full ioread64/iowrite64 implementation

The previous patch introduced ioread64/iowrite64 declarations, but
this means we no longer get the io-64-nonatomic variant, and
run into a long error when someone actually wants to use these:

ERROR: modpost: "ioread64" [drivers/net/ethernet/freescale/enetc/fsl-enetc.ko] undefined!

Add the (hopefully) correct implementation for each machine type,
based on the 32-bit accessor. Since the 32-bit return type does
not work for ioread64(), change the internal implementation to use
the correct width consistently, but leave the external interface
to match the asm-generic/iomap.h header that uses 32-bit or 64-bit
return values.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Fixes: 7e772dad ("alpha: Use generic <asm-generic/io.h>")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>