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/mvebu_db_armada8k_defconfig b/configs/mvebu_db_armada8k_defconfig
index 622d687..cd2942d 100644
--- a/configs/mvebu_db_armada8k_defconfig
+++ b/configs/mvebu_db_armada8k_defconfig
@@ -38,6 +38,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_ARP_TIMEOUT=200
 CONFIG_AHCI_MVEBU=y
 CONFIG_DM_I2C=y
 CONFIG_SYS_I2C_MVTWSI=y