[PARISC] Make PCI_HOST_ADDR and PCI_BUS_ADDR symmetrical

Change to asm-parisc/pci.h makes the define of PCI_HOST_ADDR symmetrical
with PCI_BUS_ADDR. Also add a comment about PA_VIEW and LMMIO/ELMMIO/GMMIO.

Signed-off-by: Grant Grundler <grundler@parisc-linux.org>
Signed-off-by: Kyle McMartin <kyle@parisc-linux.org>