Move the processing pipeline of the axi_adc_decimate core to its own
sub-module. This makes it easier to simulate the processing independent of
the register map.
Also since the filter is two instances of the same logic, one for each
channel, let the new sub-module model one channel and instantiate it twice.
This allows to change the implementation without having to change the same
code twice.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>