arm-bsp-extras/trusted-firmware-a/rd-aspen: Enable tbb on rd-aspen

Enable Trusted board boot on RD-Aspen platform.

- Backport cot-dt2c and its python dependencies in meta-arm,
  which is needed to build TBB.
- Update dictionary to include pydevicetree keyword.

The following flags must be set to enable trusted board boot:
TRUSTED_BOARD_BOOT=1
GENERATE_COT=1
COT=tbbr

Verify the certificates and the signed images present in
"fip.bin" image, and U-Boot can successfully boot up
with TBB feature enabled.

Signed-off-by: Sanjana Virupakshagouda <sanjana.virupakshagouda@arm.com>
Signed-off-by: David Hu <david.hu2@arm.com>
Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>