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/colibri_imx6_defconfig b/configs/colibri_imx6_defconfig
index 8e9346e5..d85591b 100644
--- a/configs/colibri_imx6_defconfig
+++ b/configs/colibri_imx6_defconfig
@@ -14,6 +14,7 @@
 CONFIG_SYS_I2C_MXC_I2C1=y
 CONFIG_SYS_I2C_MXC_I2C2=y
 CONFIG_SYS_I2C_MXC_I2C3=y
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="imx6-colibri"
 CONFIG_SPL_TEXT_BASE=0x00908000