net: lan966x: Add FDMA functionality

Ethernet frames can be extracted or injected to or from the device's
DDR memory. There is one channel for injection and one channel for
extraction. Each of these channels contain a linked list of DCBs which
contains DB. The DCB contains only 1 DB for both the injection and
extraction. Each DB contains a frame. Every time when a frame is received
or transmitted an interrupt is generated.

It is not possible to use both the FDMA and the manual
injection/extraction of the frames. Therefore the FDMA has priority over
the manual because of better performance values.

FDMA:
iperf -c 192.168.1.1
[  5]   0.00-10.02  sec   420 MBytes   352 Mbits/sec    0 sender
[  5]   0.00-10.03  sec   420 MBytes   351 Mbits/sec      receiver

iperf -c 192.168.1.1 -R
[  5]   0.00-10.01  sec   528 MBytes   442 Mbits/sec    0 sender
[  5]   0.00-10.00  sec   524 MBytes   440 Mbits/sec      receiver

Manual:
iperf -c 192.168.1.1
[  5]   0.00-10.02  sec  93.8 MBytes  78.5 Mbits/sec    0 sender
[  5]   0.00-10.03  sec  93.8 MBytes  78.4 Mbits/sec      receiver

ipers -c 192.168.1.1 -R
[  5]   0.00-10.03  sec   121 MBytes   101 Mbits/sec    0 sender
[  5]   0.00-10.01  sec   118 MBytes  99.0 Mbits/sec      receiver

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>