net: ravb: Add dependency on CONFIG_BITBANGMII

The Renesas RAVB driver always requires bitbang MDIO bus support.

Fixes: 8ae51b6f324e ("net: ravb: Add Renesas Ethernet RAVB driver")
Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 1563404..b9d8972 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -852,6 +852,7 @@
 config RENESAS_RAVB
 	bool "Renesas Ethernet AVB MAC"
 	depends on RCAR_64
+	select BITBANGMII
 	select PHYLIB
 	select PHY_ETHERNET_ID
 	help