Add capability to SPIBUS to have child device with IRQ.
For example many ADC chip have a dedicated pin to signal "data ready"
and the host can just wait for a interrupt to go out and read the result.
It is the same code as in R282674 and R282702 for IICBUS by Michal Meloun
Submitted by: Oskar Holmund <oskar.holmlund@ohdata.se>
Differential Revision: https://reviews.freebsd.org/D27396
4dd8db62e9