acpi_spmc: Register SPMC suspend/resume routines

SPMC suspend runs after the device tree is suspended using the
acpi_post_dev_suspend eventhandler, and SPMC resume runs before the
device tree is resumed using the acpi_pre_dev_suspend eventhandler.

Reviewed by: olce
Approved by: olce
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48735