ARM: zynq: disable CONFIG_SYS_MALLOC_F to fix MMC boot

Since commit 326a6823 (malloc_f: enable SYS_MALLOC_F by default
if DM is on), Zynq MMC boot hangs up after printing the following:

    U-Boot SPL 2015.04-rc5-00053-gadcc570 (Apr 08 2015 - 12:59:11)
    mmc boot
    reading system.dtb

Prior to commit 326a6823, Zynq boards enabled CONFIG_DM, but
not CONFIG_SYS_MALLOC_F.  That commit forcibly turned on
CONFIG_SYS_MALLOC_F.  I have not figured out the root cause, but
anyway it looks like CONFIG_SYS_MALLOC_F gave a bad impact on the
Zynq MMC boot.

We are planning to have the v2015.04 release in a few days.
I know this is a defensive fixup, but what I can do now is to add
   # CONFIG_SYS_MALLOC_F is not set
to every Zynq defconfig file to get back the original behavior.

Tested on:
  - Zedboard
  - ZC706 board

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Cc: Simon Glass <sjg@chromium.org>