sparc: move the leon PCI memory space comment to <asm/leon.h>

It has nothing to do with the content of the pci.h header.

Suggested by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: David S. Miller <davem@davemloft.net>