cmd: part: add explicit dependency on PARTITIONS

This is a follow-up patch for my "disk: don't compile in partition
support for spl/tpl if not really necessary".

"part" command is useful only if, at least, one of partition table types
is selected. So it should have a dependency on PARTITIONS which is now
automatically selected if one of partition table types is enabled.

With this change, *_defconfig which explicitly selects CMD_PART but
has no partition table types enabled should also be fixed.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>