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/db-88f6720_defconfig b/configs/db-88f6720_defconfig
index 2e8f4c0..72af1ec 100644
--- a/configs/db-88f6720_defconfig
+++ b/configs/db-88f6720_defconfig
@@ -43,6 +43,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_SPI_MAX_HZ=50000000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_ARP_TIMEOUT=200
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SPL_OF_TRANSLATE=y
 CONFIG_SYS_I2C_LEGACY=y