ATMEGA8U2 symbol improved

* XTAL1 set to input pin type
* UVCC, D+, D-, UGND pins rearranged
* RESET, SS, HWB, CTS, RTS overscored in pin description
* QFN-32-1EP added to footprint list
Aliases ATMEGA16U2 and ATMEGA32U2 added to ATMEGA8U2