acpi: Add an interface to obtain DSM information

The Device Specific Method (_DSM) is on optional object that defines
device specific controls. This will be useful for our power management
controller in upcoming patches. More information can be found in ACPI
spec 6.2 section 9.1.1

https://www.uefi.org/sites/default/files/resources/ACPI_6_2.pdf

This patch had a minor modification changing ENOMEM to AE_NO_MEMORY
after it got review and approval but before committing.

Test Plan: Tested in my s0ix branch

Reviewed by: kib
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D17121