Convert CONFIG_NET_RETRY_COUNT to Kconfig

This converts the following to Kconfig:
   CONFIG_NET_RETRY_COUNT

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/sama7g5ek_mmc1_defconfig b/configs/sama7g5ek_mmc1_defconfig
index 96607ee..a491268 100644
--- a/configs/sama7g5ek_mmc1_defconfig
+++ b/configs/sama7g5ek_mmc1_defconfig
@@ -45,6 +45,7 @@
 CONFIG_ENV_FAT_DEVICE_AND_PART="1:1"
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ARP_TIMEOUT=200
+CONFIG_NET_RETRY_COUNT=50
 CONFIG_DM=y
 CONFIG_CLK=y
 CONFIG_CLK_CCF=y