Partition driver requires the "PLAT_PARTITION_MAX_ENTRIES" definition.

By default, it's defined to 128 in partition driver. But it costs a lot
of memory, and only a few partition entries are really used in platform
partition table. If user wants use memory efficiently, user should
define the build flag in platform.mk instead.
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>