mx6sabreauto: set SYS_MALLOC_F for video

Sabre Auto boards currently hang with:
```
U-Boot 2019.07-rc3-00057-gc41940c406 (Jun 03 2019 - 14:42:41 +0200)

CPU:   Freescale i.MX6QP rev1.0 996 MHz (running at 792 MHz)
CPU:   Automotive temperature grade (-40C to 125C)Reset cause: WDOG
Model: Freescale i.MX6 Quad Plus SABRE Automotive Board
Board: MX6Q-Sabreauto revA
I2C:   ready
DRAM:  2 GiB
Video device 'ipu@2400000' cannot allocate frame buffer memory -ensure the device is set up before relocation
Error binding driver 'ipuv3_video': -28
Video device 'ipu@2800000' cannot allocate frame buffer memory -ensure the device is set up before relocation
Error binding driver 'ipuv3_video': -28
Some drivers failed to bind
Error binding driver 'generic_simple_bus': -28
Some drivers failed to bind
initcall sequence 8ffe00b8 failed at call 1780e93b (err=-28)
```

Set SYS_MALLOC_F_LEN to reserve_video to work.

This is similar to the change Peng Fan did for mx6sabresd (9002e735)
Signed-off-by: NSjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: NPeng Fan <peng.fan@nxp.com>

configs/mx6sabreauto_defconfig

@@ -4,6 +4,7 @@ CONFIG_SYS_TEXT_BASE=0x17800000
 CONFIG_SPL_GPIO_SUPPORT=y
 CONFIG_SPL_LIBCOMMON_SUPPORT=y
 CONFIG_SPL_LIBGENERIC_SUPPORT=y
+CONFIG_SYS_MALLOC_F_LEN=0x4000
 CONFIG_TARGET_MX6SABREAUTO=y
 CONFIG_SPL_MMC_SUPPORT=y
 CONFIG_SPL_SERIAL_SUPPORT=y
@@ -11,7 +12,6 @@ CONFIG_NR_DRAM_BANKS=1
 CONFIG_SPL=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_NXP_BOARD_REVISION=y
-# CONFIG_SYS_MALLOC_F is not set
 CONFIG_FIT=y
 CONFIG_SPL_FIT_PRINT=y
 CONFIG_SPL_LOAD_FIT=y