Using interface definitions makes it possible to group pins of a peripheral
into a interface pins. This allows us to use connect_bd_intf_net to connect
all pins of the interface instead of having to manually call connect_bd_net
for each for the pins.
Using interface pins also unclutters the connections in the Vivado block
design view a bit.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>