Use EARLY_DRIVER_MODULE in the acpi bus.

We need this to use EARLY_DRIVER_MODULE in child drivers on arm64. This
should be a no-op on x86 as it has DRIVER_MODULE in the nexus driver making
all later drivers attach in the last pass.

Reviewed by: imp
MFC after: 1 month
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D23717