ARM: mvebu: set CONFIG_SYS_MALLOC_F_LEN to 0x2000

Set CONFIG_SYS_MALLOC_F_LEN to match the rest of the mvebu boards. The
default of 0x400 is not enough when booting from SPI.
Signed-off-by: NChris Packham <judge.packham@gmail.com>
Signed-off-by: NStefan Roese <sr@denx.de>

configs/db-88f6820-amc_defconfig

@@ -3,6 +3,7 @@ CONFIG_ARCH_MVEBU=y
 CONFIG_SYS_TEXT_BASE=0x00800000
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_TARGET_DB_88F6820_AMC=y
 CONFIG_SPL_SERIAL_SUPPORT=y
 CONFIG_SPL=y