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/nyan-big_defconfig b/configs/nyan-big_defconfig
index 3e328d9..9d7d54e 100644
--- a/configs/nyan-big_defconfig
+++ b/configs/nyan-big_defconfig
@@ -4,6 +4,7 @@
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_OFFSET=0xFFFFE000
+CONFIG_SYS_MALLOC_LEN=0x2500000
 CONFIG_DEFAULT_DEVICE_TREE="tegra124-nyan-big"
 CONFIG_SPL_TEXT_BASE=0x80108000
 CONFIG_BOOTSTAGE_STASH_ADDR=0x83000000