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/taurus_defconfig b/configs/taurus_defconfig
index 4eda59a..35f7c44 100644
--- a/configs/taurus_defconfig
+++ b/configs/taurus_defconfig
@@ -18,6 +18,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_OFFSET=0x100000
+CONFIG_SYS_MALLOC_LEN=0x460000
 CONFIG_SPL_DM_SPI=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g20-taurus"
 CONFIG_SPL_SYS_MALLOC_F_LEN=0x1000