Power saving

[SukkoPera]

The following lines should turn off a few unused AVR peripherals, helping us save some power. I don't know how effective the saving will be, but I think it's a good idea to try and minimize our current draw, as some PSX controllers might already be quite demanding:

ADCSRA &= ~(1 << ADEN);  // Turn off ADC
ACSR |= (1 << ACD); // Turn off Analog Comparator
DIDR1 |= (1 << AIN1D) | (1 << AIN0D); // Turn off AIN digital input buffer
PRR |= (1 << PRTWI) | \  // Turn off TWI/I2C Interface
       (1 << PRTIM2) | \  // Turn off Timer 2
       (1 << PRTIM1) | \  // Turn off Timer 1
       (1 << PRSPI) | \  // Turn off SPI Interface
#ifndef ENABLE_SERIAL_DEBUG
       (1 << PRUSART0) | \  // Turn off Serial Port
#endif
       (1 << PRADC);   // Turn off ADC

The last command might make the others redundant. I guess we should take some measures.