media: allegro: add encoder buffer support

The encoder buffer serves as a cache for reference frames during the
encoding process. The encoder buffer significantly reduces the bandwidth
requirement for read accesses on the AXI ports of the VCU, but slightly
reduces the quality of the encoded video.

The encoder buffer must be configured as a whole during the firmware
initialization and later explicitly enabled for every channel that shall
use the encoder buffer.

Prior to firmware version 2019.2, it was necessary to explicitly set the
size of the encoder buffer for every channel. Since 2019.2 it is
sufficient to enable the encoder buffer and leave the rest to the
firmware. Therefore, only support the encoder buffer for firmware 2019.2
and later.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

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