[magma] Import AMD's MIT-licensed Linux driver.

This is a pristine copy, with the GPL-licensed build configs removed.
A LICENSE and METADATA file have been added, per the third-party
guidelines.

This driver coordinates with the AMD-VLK ICD, which will be imported
in a separate change. Both are permissively licensed, but need to be
ported from Linux to Fuchsia.

Source: "v5.3" tag from github.com/torvalds/linux
Change-Id: I0b4c2fc58e556d6923d00c4c3933a55b4febeb19