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/mx53cx9020_defconfig b/configs/mx53cx9020_defconfig
index ef6bd0f..4798510 100644
--- a/configs/mx53cx9020_defconfig
+++ b/configs/mx53cx9020_defconfig
@@ -20,6 +20,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_MMC=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_ARP_TIMEOUT=200
 CONFIG_FPGA_ALTERA=y
 CONFIG_FPGA_CYCLON2=y
 CONFIG_FSL_ESDHC_IMX=y