openpic: symbolicize some magic numbers

openpic: symbolicize some magic numbers

Deefine symbolic names for some register bits, and use some that
have already been defined.

Also convert some register values from hex to decimal when it improves
readability.

IPVP_PRIORITY_MASK is corrected from (0x1F << 16) to (0xF << 16), in
conjunction with making wider use of the symbolic name.  I looked at
Freescale and IBM MPIC docs and at the base OpenPIC spec, and all three
had priority as 4 bits rather than 5.  Plus, the magic nubmer that is
being replaced with symbolic values treated the field as 4 bits wide.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>