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/T1024RDB_SDCARD_defconfig b/configs/T1024RDB_SDCARD_defconfig
index 8d3f6a7..1b67d27 100644
--- a/configs/T1024RDB_SDCARD_defconfig
+++ b/configs/T1024RDB_SDCARD_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SYS_MEMTEST_END=0x00400000
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0x100000
+CONFIG_SYS_MALLOC_LEN=0xa00000
 CONFIG_DEFAULT_DEVICE_TREE="t1024rdb"
 CONFIG_SPL_TEXT_BASE=0xFFFD8000
 CONFIG_SPL_MMC_SUPPORT=y