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/bcm_ns3_defconfig b/configs/bcm_ns3_defconfig
index 469736d..3fa2435 100644
--- a/configs/bcm_ns3_defconfig
+++ b/configs/bcm_ns3_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_TEXT_BASE=0xFF000000
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SIZE=0x80000
+CONFIG_SYS_MALLOC_LEN=0xc00000
 CONFIG_DEFAULT_DEVICE_TREE="ns3-board"
 CONFIG_SYS_LOAD_ADDR=0x80080000
 CONFIG_FIT=y