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_rtconly_defconfig b/configs/am43xx_evm_rtconly_defconfig
index 30e44cb..6b24aac 100644
--- a/configs/am43xx_evm_rtconly_defconfig
+++ b/configs/am43xx_evm_rtconly_defconfig
@@ -22,6 +22,7 @@
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SPL_FALCON_BOOT_MMCSD=y
 CONFIG_SYS_MMCSD_RAW_MODE_KERNEL_SECTOR=0x1700
+CONFIG_SYS_MAXARGS=64
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00100000
 CONFIG_CMD_SPL_WRITE_SIZE=0x40000