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/sam9x60ek_mmc_defconfig b/configs/sam9x60ek_mmc_defconfig
index 0b865b7..b262332 100644
--- a/configs/sam9x60ek_mmc_defconfig
+++ b/configs/sam9x60ek_mmc_defconfig
@@ -6,6 +6,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x8000
 CONFIG_NR_DRAM_BANKS=8
 CONFIG_ENV_SIZE=0x4000
+CONFIG_SYS_MALLOC_LEN=0x81000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="sam9x60ek"
 CONFIG_DEBUG_UART_BOARD_INIT=y