counter: Update documentation for new counter registration functions

and Counts, and to set and get the "action mode" and "function mode" for

various Synapses and Counts respectively.

A defined counter_device structure may be registered to the system by

passing it to the counter_register function, and unregistered by passing

it to the counter_unregister function. Similarly, the

devm_counter_register function may be used if device memory-managed

registration is desired.

A counter_device structure is allocated using counter_alloc() and then

registered to the system by passing it to the counter_add() function, and

unregistered by passing it to the counter_unregister function. There are

device managed variants of these functions: devm_counter_alloc() and

devm_counter_add().

The struct counter_comp structure is used to define counter extensions

for Signals, Synapses, and Counts.