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/r8a779a0_falcon_defconfig b/configs/r8a779a0_falcon_defconfig
index 9fce6c2..c1685aa 100644
--- a/configs/r8a779a0_falcon_defconfig
+++ b/configs/r8a779a0_falcon_defconfig
@@ -57,6 +57,7 @@
 CONFIG_SPI_FLASH_SPANSION=y
 # CONFIG_SPI_FLASH_USE_4K_SECTORS is not set
 CONFIG_BITBANGMII=y
+CONFIG_BITBANGMII_MULTI=y
 CONFIG_PHY_MICREL=y
 CONFIG_PHY_MICREL_KSZ90X1=y
 CONFIG_DM_ETH=y