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/rzg2_beacon_defconfig b/configs/rzg2_beacon_defconfig
index 91b3fa2..5babd48 100644
--- a/configs/rzg2_beacon_defconfig
+++ b/configs/rzg2_beacon_defconfig
@@ -62,6 +62,7 @@
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_BITBANGMII=y
+CONFIG_BITBANGMII_MULTI=y
 CONFIG_PHY_ATHEROS=y
 CONFIG_DM_ETH=y
 CONFIG_RENESAS_RAVB=y