disk: don't compile in partition support for spl/tpl if not really necessary

Under the current Kconfigs, disk/part.c will be compiled in even if none of
partition table types are enabled. This will lead to the size growth of SPL
/TPL code.

With this patch, CONFIG_PARTITIONS is selected only if, at least, one of
CONFIG_*_PARTITION is enabled.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>