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/nitrogen6dl2g_defconfig b/configs/nitrogen6dl2g_defconfig
index aefa573..e9282c3 100644
--- a/configs/nitrogen6dl2g_defconfig
+++ b/configs/nitrogen6dl2g_defconfig
@@ -15,6 +15,7 @@
 CONFIG_SYS_I2C_MXC_I2C2=y
 CONFIG_SYS_I2C_MXC_I2C3=y
 CONFIG_DDR_MB=2048
+CONFIG_SYS_MALLOC_LEN=0xa00000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx6dl-nitrogen6x"
 CONFIG_CMD_HDMIDETECT=y