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/p2571_defconfig b/configs/p2571_defconfig
index 0cf4ddc..27cc68e 100644
--- a/configs/p2571_defconfig
+++ b/configs/p2571_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="tegra210-p2571"
 CONFIG_TEGRA210=y
 CONFIG_TARGET_P2571=y