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/am65x_evm_a53_defconfig b/configs/am65x_evm_a53_defconfig
index 9f41b39..3c65439 100644
--- a/configs/am65x_evm_a53_defconfig
+++ b/configs/am65x_evm_a53_defconfig
@@ -55,6 +55,7 @@
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_DFU=y
 CONFIG_SPL_YMODEM_SUPPORT=y
+CONFIG_SYS_MAXARGS=64
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_DFU=y
 CONFIG_CMD_GPT=y