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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 904fc53..184e886 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -30,6 +30,7 @@
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x1700
 # CONFIG_SPL_POWER is not set
 CONFIG_SYS_PROMPT="AM3517_EVM # "
+CONFIG_SYS_MAXARGS=64
 # CONFIG_CMD_IMI is not set
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0xaa0000