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/sama7g5ek_mmc_defconfig b/configs/sama7g5ek_mmc_defconfig
index 7abd5c8..6c25e01 100644
--- a/configs/sama7g5ek_mmc_defconfig
+++ b/configs/sama7g5ek_mmc_defconfig
@@ -44,6 +44,7 @@
 CONFIG_ENV_IS_IN_FAT=y
 CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_ARP_TIMEOUT=200
 CONFIG_DM=y
 CONFIG_CLK=y
 CONFIG_CLK_CCF=y