package/linux-firmware: install AMD Radeon firmware

Signed-off-by: David BARBION <david.barbion@ext.leroymerlin.fr>
[yann.morin.1998@free.fr:
  - simplify by copying the full directory
  - fix title
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>