Add FECFLAG_AVB variant flag to support new features on imx7.

This flag is analogous to the Linux driver FEC_QUIRK_HAS_AVB. It
indicates an FEC with support for Audio Video Bridging (AVB). This
indicator is used for various other parts in the Linux driver
(drivers/net/ethernet/freescale/fec_main.c).

Use it to customize the receive/transmit buffer alignment. The receive
buffer alignment increased to 64-bytes on the i.MX 6SoloX and i.MX
7Dual. There are no hard alignment restrictions for transmit buffers on
these chips.

Fix the ffec_softc::fectype type to provide enough storage for the
feature flags.

PR: 222634
Submitted by: sebastian.huber@embedded-brains.de