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/imx8mn_var_som_defconfig b/configs/imx8mn_var_som_defconfig
index 5ec82f2..d4930b1 100644
--- a/configs/imx8mn_var_som_defconfig
+++ b/configs/imx8mn_var_som_defconfig
@@ -35,6 +35,7 @@
 CONFIG_SPL_POWER=y
 CONFIG_SPL_WATCHDOG=y
 CONFIG_SYS_PROMPT="> "
+CONFIG_SYS_MAXARGS=64
 CONFIG_CMD_FUSE=y
 CONFIG_CMD_MMC=y
 CONFIG_CMD_USB=y