pci: Indicate prefetchable memory allocate when debugging

At present the PCI output displays 'Mem' when it allocates memory for a PCI
device, whether it is prefetchable or not. There is a distinction since the
memory comes from separate pools. Use 'Prf' instead of 'Mem' when allocating
prefetchable memory.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>