Correct ATTiny88 pins_arduino.h

Made corrections to the pins_arduino.h file for the ATTiny88:
(1) Included PORTA as some parts have this.
(2) Corrected #defines for number of digital and analog pins
(3) Corrected pin change interrupt mappings.
(4) Corrected analog pin (A0 etc) constants.
(5) Corrected diagram for which pins have PWM.