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/smdkv310_defconfig b/configs/smdkv310_defconfig
index 8b7d46c..ba773dc 100644
--- a/configs/smdkv310_defconfig
+++ b/configs/smdkv310_defconfig
@@ -7,6 +7,7 @@
 CONFIG_ARCH_EXYNOS4=y
 CONFIG_ENV_SIZE=0x4000
 CONFIG_ENV_OFFSET=0x4200
+CONFIG_SYS_MALLOC_LEN=0x5004000
 CONFIG_DEFAULT_DEVICE_TREE="exynos4210-smdkv310"
 CONFIG_SPL_TEXT_BASE=0x02021410
 CONFIG_SPL=y