QOMified the pflash_cfi0x so machine models can connect them up in custom ways.

Kept the pflash_cfi0x_register functions as is. They can still be used to
create a flash straight onto system memory.

Signed-off-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>