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_nandflash_defconfig b/configs/sam9x60ek_nandflash_defconfig
index 7508784..14a8094 100644
--- a/configs/sam9x60ek_nandflash_defconfig
+++ b/configs/sam9x60ek_nandflash_defconfig
@@ -5,6 +5,7 @@
 CONFIG_TARGET_SAM9X60EK=y
 CONFIG_SYS_MALLOC_F_LEN=0x8000
 CONFIG_NR_DRAM_BANKS=8
+CONFIG_SYS_MALLOC_LEN=0x81000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="sam9x60ek"
 CONFIG_DEBUG_UART_BOARD_INIT=y