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/corvus_defconfig b/configs/corvus_defconfig
index 2646cf5..2d817bc 100644
--- a/configs/corvus_defconfig
+++ b/configs/corvus_defconfig
@@ -13,6 +13,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x800
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_OFFSET=0x100000
+CONFIG_SYS_MALLOC_LEN=0x460000
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9g45-corvus"
 CONFIG_SPL_TEXT_BASE=0x300000
 CONFIG_SPL_SERIAL_SUPPORT=y