Convert CONFIG_SYS_MALLOC_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MALLOC_LEN

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/am65x_evm_r5_usbdfu_defconfig b/configs/am65x_evm_r5_usbdfu_defconfig
index be68a4f..c64f72f 100644
--- a/configs/am65x_evm_r5_usbdfu_defconfig
+++ b/configs/am65x_evm_r5_usbdfu_defconfig
@@ -9,6 +9,7 @@
 CONFIG_K3_EARLY_CONS=y
 CONFIG_TARGET_AM654_R5_EVM=y
 CONFIG_ENV_SIZE=0x20000
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="k3-am654-r5-base-board"
 CONFIG_SPL_TEXT_BASE=0x41c00000