drivers/pcie/generic: Clean up driver

This removes unneeded and unused functionality in the driver as part of
an effort to make the driver more generic and useful. The things that
have been removed are: `DmaProperty` and its associated `is_untrusted`
config, `_DSD` generation, and the companion device functionality. This
driver isn't currently used anywhere so there won't be any issues from
removing the above functionality.

BUG=b:237682766
TEST=Built and booted coreboot on Skyrim device

Change-Id: I0abd9148ab66ea9426069102ecc8c2fa77fea98e
Signed-off-by: Robert Zieba <robertzieba@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/65797
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

0 files changed, 0 insertions, 0 deletions