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/cortina_presidio-asic-base_defconfig b/configs/cortina_presidio-asic-base_defconfig
index 84c8743..04a9934 100644
--- a/configs/cortina_presidio-asic-base_defconfig
+++ b/configs/cortina_presidio-asic-base_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_TEXT_BASE=0x04000000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x20000
+CONFIG_SYS_MALLOC_LEN=0x820000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="ca-presidio-engboard"
 CONFIG_IDENT_STRING="Presidio-SoC"