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_hs_evm_defconfig b/configs/am43xx_hs_evm_defconfig
index 1517b70..cc75fe9 100644
--- a/configs/am43xx_hs_evm_defconfig
+++ b/configs/am43xx_hs_evm_defconfig
@@ -33,6 +33,7 @@
 CONFIG_SPL_USB_STORAGE=y
 CONFIG_SPL_USB_GADGET=y
 CONFIG_SPL_USB_ETHER=y
+CONFIG_SYS_MAXARGS=64
 CONFIG_SYS_I2C_EEPROM_ADDR_LEN=2
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_NAND=y