"Do you know a dedicated IC that has only the count (in both direction) features"?
You may be surprised to learn that a few simple, cheap, generic logic chips are sufficient to decode a rotary encoder -- it doesn't require some special chip specifically customized for rotary decoders.
The Codewheel Generator Page
has a few relatively simple circuits that decode the 2 rotary quadrature outputs into a "direction" and "count" outputs.
Those outputs can be wired into the "direction" and "clock" inputs of any up/down counter -- such as, for example, the CD4029, the CD4516, the CD40193, 74HC193, etc.
"is it possible to make à simple one with an attiny"?
Yes, you can make a simple quadrature encoder with an ATtiny.
Some tips for connecting a quadrature encoder to a Atmel chip and decoding it in software are at:
If you have relatively few pulses-per-second, you might be able to run the 2 signals from each encoder directly into your main ATmega and use the above techniques to do it all in software, without any external conversion hardware.