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/espresso7420_defconfig b/configs/espresso7420_defconfig
index 55671ab..fc96881 100644
--- a/configs/espresso7420_defconfig
+++ b/configs/espresso7420_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_NR_DRAM_BANKS=8
 CONFIG_ENV_SIZE=0x4000
+CONFIG_SYS_MALLOC_LEN=0x5004000
 CONFIG_DEFAULT_DEVICE_TREE="exynos7420-espresso7420"
 CONFIG_IDENT_STRING=" for ESPRESSO7420"
 CONFIG_SYS_LOAD_ADDR=0x43e00000