No fixed value for USB power current

This should save some cycles inside ISR Handler.

Change the way ISRs are stored. Store now in a list on a ‘first come
first served basis’, the first interrupt that gets attached will be
served first. This improves the speed of the ISR calling from about 6us
to 2us for a single interrupt on extint14

See #200

Solve #199

Fix #223

See https://github.com/arduino/Arduino/issues/4525

PWM frequency 732 Hz

The RXC interrupt was being disabled when using bit to disable the DRE
interrupt.

These functions were changed from private to protected but the comments were not updated at that time.

In conjunction with equivalent pull requests to Arduino AVR Boards and Arduino SAM Boards, solves #6146.

Add platform specific keywords.txt