at91: enable Micrel PHY driver

The AT91 SAMA5D3XPLAINED uses a KSZ9031 and a KSZ8081 that are supported
by this PHY driver.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46171 3c298f89-4303-0410-b956-a3cf2f4a3e73

target/linux/at91/config-default

@@ -120,6 +120,7 @@ CONFIG_MACB=y
 CONFIG_MACH_AT91SAM9_DT=y
 CONFIG_MDIO_BOARDINFO=y
 # CONFIG_MII is not set
+CONFIG_MICREL_PHY=y
 CONFIG_MMC=y
 CONFIG_MMC_ATMELMCI=y
 CONFIG_MMC_BLOCK=y