CPU: Allow multiple drivers for the RDY line

[dirkwhoffmann]

To improve the accuracy of REU emulation, the REU needs to pull down the CPU's RDY line at the beginning of a DMA transfer and release it afterward. Currently, the RDY line is modeled by a boolean, meaning the REU would conflict with VICII.

TODO: Allow multiple sources to pull the RDY line low. Such a functionality already exists for the IRQ line and the NMI line. Implement the RDY line accordingly.