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/smdk5420_defconfig b/configs/smdk5420_defconfig
index a759199..fa0c2ea 100644
--- a/configs/smdk5420_defconfig
+++ b/configs/smdk5420_defconfig
@@ -10,6 +10,7 @@
 CONFIG_ENV_SIZE=0x4000
 CONFIG_ENV_OFFSET=0x3FC000
 CONFIG_ENV_SECT_SIZE=0x4000
+CONFIG_SYS_MALLOC_LEN=0x5004000
 CONFIG_DEFAULT_DEVICE_TREE="exynos5420-smdk5420"
 CONFIG_SPL_TEXT_BASE=0x02024410
 CONFIG_SPL=y