cores: add spi declaration

convert spi structure definitions to AVR_SPI_DECLARE

modified:   simavr/cores/sim_90usb162.c
modified:   simavr/cores/sim_mega128.c
modified:   simavr/cores/sim_mega1280.c
modified:   simavr/cores/sim_mega1281.c
modified:   simavr/cores/sim_mega128rfr2.c
modified:   simavr/cores/sim_mega169.c
modified:   simavr/cores/sim_mega2560.c
modified:   simavr/cores/sim_megax.h
modified:   simavr/cores/sim_megax4.h
modified:   simavr/cores/sim_megax8.h
modified:   simavr/cores/sim_megaxm1.h
modified:   simavr/sim/avr_spi.h