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/gardena-smart-gateway-at91sam_defconfig b/configs/gardena-smart-gateway-at91sam_defconfig
index e8efc84..5946e23 100644
--- a/configs/gardena-smart-gateway-at91sam_defconfig
+++ b/configs/gardena-smart-gateway-at91sam_defconfig
@@ -11,6 +11,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x10000
+CONFIG_SYS_MALLOC_LEN=0x1000000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g25-gardena-smart-gateway"
 CONFIG_SPL_TEXT_BASE=0x300000