ar71xx: Allow to set the RXDV, RXD, TXD, TXE delays for QCA955x

ar71xx: Allow to set the RXDV, RXD, TXD, TXE delays for QCA955x

Some u-boot versions for QCA955x change the delays based on the link speed
during boot. This usually breaks the support of other linkspeeds when
OpenWrt is booted. It also conflicts with the
at803x_platform_data::fixup_rgmii_tx_delay. OpenWrt has to set its own
values in QCA955X_GMAC_REG_ETH_CFG.

The default RGMII values from the Atheros u-boot are currently used to
preset the existing mach files. These may have to be adjusted for boards
using different values but which are not currently set them explicitely in
OpenWrt.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: Imre Kaloz <kaloz@openwrt.org>
Cc: Christian Beier <cb@shoutrlabs.com>
Cc: Chris R Blake <chrisrblake93@gmail.com>
Cc: Benjamin Berg <benjamin@sipsolutions.net>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: Cezary Jackiewicz <cezary.jackiewicz@gmail.com>
Cc: Matthias Schiffer <mschiffer@universe-factory.net>
Cc: Dirk Neukirchen <dirkneukirchen@web.de>
Cc: Christian Mehlis <christian@m3hlis.de>
Cc: Luka Perkov <luka@openwrt.org>
Cc: Felix Fietkau <nbd@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@49029 3c298f89-4303-0410-b956-a3cf2f4a3e73