WIP: Raise SPI_IRQ_OUTPUT IRQ in SPI slave mode

[aznoohwee]

For #296 and #222, raise IRQ when firmware is operating as a SPI slave.

Nice to haves to make this PR more complete:

• Unit test for SPI slave
• Check status of !SS line
• Only raise master IRQ if SPIE is enabled

[buserror]

Could you please rebase so there are no reverted comments? I suggest git rebase -i -- and squash stuff that is relevant together

[aznoohwee]

Thanks and done!

[aznoohwee]

Placing a hold on PR. This is not quite the correct behavior. SPI ISR should be raised when data is received from master, not on slave register write.

[aznoohwee]

unit test written and module checks out. no need to change, but leaving PR open if you want to include test.