GICv2m: parse SPI_BASE and NUM_SPIS from DTS

[kd]

Summary

If DTS contains values for SPI base/count, use these instead of parsing TYPER register.
Some architectures (e.g. BCM NS2) are using DTS to fix hardware issues.

[andrew]

Can you include the context, e.g. reupload with one of the commands at https://wiki.freebsd.org/action/show/Phabricator#Create_a_Revision_via_Web_Interface