spibus(4): Add support for ACPI-based children enumeration

When spibus is attached as child of Intel SPI controller it scans all
ACPI nodes for "SPI Serial Bus Connection Resource Descriptor" described
in section 19.6.126 of ACPI specs.
If such a descriptor is found, SPI child is added to spibus, it's SPI
chip select, mode, clock, IRQ resource and ACPI handle are added to ivars.
Existing ACPI bus-hosted child is deleted afterwards.
Apple ACPI SPI extensions are supported.

Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D41248