sunxi: Fix gmac not working reliable on the Bananapi

In order for the gmac nic to work reliable on the Bananapi, we need to set
bits 10-12 GTXDC "GMAC Transmit Clock Delay Chain" of the GMAC clk register
(0x01c20164) to 3.

Without this about 9 out of 10 ethernet packets get lost, with this setting
there is no packet loss.

So far setting these bits is only necessary on the Bananapi, so this commit
solves this with a bit of #ifdef CONFIG_BANANAPI code. If in the future we
need to do something similar for other boards, we can create a specific
CONFIG_FOO option for this then.

Reported-by: Karsten Merker <merker@debian.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Karsten Merker <merker@debian.org>
Tested-by: Zoltan HERPAI <wigyori@openwrt.org>
Tested-by: Tony Zhang <tony.zhang@lemaker.org>
Acked-by: Ian Campbell <ijc@hellion.org.uk>