Convert CONFIG_BITBANGMII_MULTI to Kconfig

This converts the following to Kconfig:
   CONFIG_BITBANGMII_MULTI

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/silk_defconfig b/configs/silk_defconfig
index 6ad3252..fe28514 100644
--- a/configs/silk_defconfig
+++ b/configs/silk_defconfig
@@ -76,6 +76,7 @@
 CONFIG_SPI_FLASH_SPANSION=y
 CONFIG_SPI_FLASH_MTD=y
 CONFIG_BITBANGMII=y
+CONFIG_BITBANGMII_MULTI=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ8XXX=y
 CONFIG_DM_ETH=y