Convert CONFIG_ARP_TIMEOUT to Kconfig

This converts the following to Kconfig:
   CONFIG_ARP_TIMEOUT

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/socfpga_is1_defconfig b/configs/socfpga_is1_defconfig
index 01b7086..9ad37a0 100644
--- a/configs/socfpga_is1_defconfig
+++ b/configs/socfpga_is1_defconfig
@@ -41,6 +41,7 @@
 CONFIG_USE_BOOTFILE=y
 CONFIG_BOOTFILE="zImage"
 CONFIG_VERSION_VARIABLE=y
+CONFIG_ARP_TIMEOUT=500
 CONFIG_SPL_DM_SEQ_ALIAS=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_DWAPB_GPIO=y