Revert "ARM: zynq: disable CONFIG_SYS_MALLOC_F to fix MMC boot"

This reverts commit 321f86e1.

The original bug has been fixed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-on: Zedboard and ZC706 board
Tested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-on: zc702
Tested-by: Michal Simek <michal.simek@xilinx.com>