ar71xx: add a helper function to set RXDV/RXD of ETH_CFG on AR934x

ar71xx: add a helper function to set RXDV/RXD of ETH_CFG on AR934x

The ETH_RXDV_DELAY (17:16) and ETH_RXD_DELAY (15:14) are currently not cleared
by the function ath79_setup_ar934x_eth_cfg. Clearing these in the
ath79_setup_ar934x_eth_cfg may cause problems on some hardware because they
rely on the preset value by the bootloader.

Instead another function is introduced which also works on ETH_CFG on AR934x.
It can be used to safely clear and set ETH_RXDV_DELAY and ETH_RXD_DELAY on
machines which require special settings.

Signed-off-by: Sven Eckelmann <sven@open-mesh.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@45523 3c298f89-4303-0410-b956-a3cf2f4a3e73