Add support for T3.5

[crteensy]

Teensy 3.5's SPI1 and SPI2 only have one DMA channel trigger source shared by RX and TX. How these can be used for DMASPI is still not quite clear. Threads with most information:

• https://forum.pjrc.com/threads/43585-Teensy-3-5-SPI-DMA
• https://forum.pjrc.com/threads/35760-DMASPI-library-needs-some-(probably-breaking)-changes-to-really-support-multiple-SPIs?p=142449&viewfull=1#post142449