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/am43xx_evm_defconfig b/configs/am43xx_evm_defconfig
index 7a736b6..6e37c2c 100644
--- a/configs/am43xx_evm_defconfig
+++ b/configs/am43xx_evm_defconfig
@@ -28,6 +28,7 @@
 CONFIG_SPL_USB_HOST=y
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_USB_ETHER=y
+CONFIG_SYS_MAXARGS=64
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00100000
 CONFIG_CMD_SPL_WRITE_SIZE=0x40000