add IS_PIN_SPI(p) to Boards.h

I've added a macro 'IS_PIN_SPI(p)' to Boards.h.
While SPI is not yet a feature in Firmata I can use this info in EthernetFirmata to exclude the SPI-pins from Capability- and PinMode-query (and hindering systemReset to set these pins to the default OUTPUT) which would interfere with the running Ethernet-lib and bring down an active IP-connection.
Since I didn't have reliable SPI-info at hand for the non-arduino boards I made IS_PIN_SPI(p) evaluate to '0' for all boards that do not have this macro defined.