Convert CONFIG_SYS_MAXARGS to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MAXARGS

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/am64x_evm_r5_defconfig b/configs/am64x_evm_r5_defconfig
index 158c43e..0f76a0a 100644
--- a/configs/am64x_evm_r5_defconfig
+++ b/configs/am64x_evm_r5_defconfig
@@ -60,6 +60,7 @@
 CONFIG_SPL_DFU=y
 CONFIG_SPL_YMODEM_SUPPORT=y
 CONFIG_HUSH_PARSER=y
+CONFIG_SYS_MAXARGS=64
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_DFU=y
 CONFIG_CMD_GPT=y