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/silinux_ek874_defconfig b/configs/silinux_ek874_defconfig
index 5b01f1e..0dafb70 100644
--- a/configs/silinux_ek874_defconfig
+++ b/configs/silinux_ek874_defconfig
@@ -57,6 +57,7 @@
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_BITBANGMII=y
+CONFIG_BITBANGMII_MULTI=y
 CONFIG_PHY_REALTEK=y
 CONFIG_DM_ETH=y
 CONFIG_RENESAS_RAVB=y