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/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig b/configs/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig
index f0e2d58..4a200b6 100644
--- a/configs/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig
+++ b/configs/ls1021atwr_sdcard_ifc_SECURE_BOOT_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SYS_I2C_MXC_I2C1=y
 CONFIG_SYS_I2C_MXC_I2C2=y
 CONFIG_SYS_I2C_MXC_I2C3=y
+CONFIG_SYS_MALLOC_LEN=0x1020000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="ls1021a-twr-duart"
 CONFIG_SPL_TEXT_BASE=0x10000000