sunxi: set GMAC TX delay = 0x1 on Cubietruck.

Of 4 boards in our automated test system 2 do not have reliable
networking with the default TX delay of 0x0. Increasing to 0x1 seems
to make things reliable on all 4 boards.

Some previous ad-hpoc tests with tx delay set to 0, 1, 2 and 3 on one
of the problematic boards showed:

0: mw.l 0x1c20164 0x006 1     -- t/o in 4/5 tftp runs
1: mw.l 0x1c20164 0x406 1     -- t/o in 1/5 tftp runs
2: mw.l 0x1c20164 0x806 1     -- t/o in 1/5 tftp runs
3: mw.l 0x1c20164 0xc06 1     -- t/o many times in first tftp run

For 0, 1 and 2 "t/o" means one or two "T" glitches in the download,
but it did complete. For 3 those were basically continuous and it
couldn't complete.

tftp was of a 16M initrd.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

configs/Cubietruck_defconfig

@@ -1,6 +1,7 @@
 CONFIG_SPL=y
 CONFIG_SYS_EXTRA_OPTIONS="AXP209_POWER,SUNXI_GMAC,RGMII,AHCI,SATAPWR=SUNXI_GPH(12),USB_EHCI"
 CONFIG_FDTFILE="sun7i-a20-cubietruck.dtb"
+CONFIG_GMAC_TX_DELAY=1
 CONFIG_VIDEO_VGA=y
 CONFIG_ARM=y
 CONFIG_ARCH_SUNXI=y