net: Add an SPL config for atheros

Add a new SPL_PHY_ATHEROS to avoid a build error on am335x_evm with split
config.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
diff --git a/drivers/net/phy/Kconfig b/drivers/net/phy/Kconfig
index 5eaff05..6806e3c 100644
--- a/drivers/net/phy/Kconfig
+++ b/drivers/net/phy/Kconfig
@@ -106,6 +106,9 @@
 config PHY_ATHEROS
 	bool "Atheros Ethernet PHYs support"
 
+config SPL_PHY_ATHEROS
+	bool "Atheros Ethernet PHYs support (SPL)"
+
 config PHY_BROADCOM
 	bool "Broadcom Ethernet PHYs support"