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/grpeach_defconfig b/configs/grpeach_defconfig
index 678dbd3..d37b48d 100644
--- a/configs/grpeach_defconfig
+++ b/configs/grpeach_defconfig
@@ -44,6 +44,7 @@
 CONFIG_SPI_FLASH_MACRONIX=y
 # CONFIG_SPI_FLASH_USE_4K_SECTORS is not set
 CONFIG_BITBANGMII=y
+CONFIG_BITBANGMII_MULTI=y
 CONFIG_DM_ETH=y
 CONFIG_SH_ETHER=y
 CONFIG_PINCTRL=y