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/sifive_unleashed_defconfig b/configs/sifive_unleashed_defconfig
index 5089764..cf50101 100644
--- a/configs/sifive_unleashed_defconfig
+++ b/configs/sifive_unleashed_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SPL_GPIO=y
 CONFIG_SYS_MALLOC_F_LEN=0x3000
 CONFIG_NR_DRAM_BANKS=1
+CONFIG_SYS_MALLOC_LEN=0x800000
 CONFIG_SPL_DM_SPI=y
 CONFIG_DEFAULT_DEVICE_TREE="hifive-unleashed-a00"
 CONFIG_SPL_MMC_SUPPORT=y