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/drivers/net/phy/Kconfig b/drivers/net/phy/Kconfig
index eed6eb1..014a4de 100644
--- a/drivers/net/phy/Kconfig
+++ b/drivers/net/phy/Kconfig
@@ -2,6 +2,10 @@
 config BITBANGMII
 	bool "Bit-banged ethernet MII management channel support"
 
+config BITBANGMII_MULTI
+	bool "Enable the multi bus support"
+	depends on BITBANGMII
+
 config MV88E6352_SWITCH
 	bool "Marvell 88E6352 switch support"