(hack): set gmac tx delay to 4 (see full desc)

Required to have working ethernet on Lime2 rev. K

Other boards require other GMAC TX delay, but the previous commit(s)
forward porting the Olimex network patch have delays hardcoded for other
revisions of the Lime2, so this should not break anything.

configs/A20-OLinuXino-Lime2_defconfig

@@ -45,3 +45,7 @@ CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_MUSB_GADGET=y
 CONFIG_SYS_USB_EVENT_POLL_VIA_INT_QUEUE=y
 CONFIG_SPL_SPI_SUNXI=y
+
+CONFIG_PHY_MICREL=y
+CONFIG_PHY_MICREL_KSZ90X1=y
+CONFIG_GMAC_TX_DELAY=4