disk: define nullified functions for !PARTITIONS

Some defconfig enables CMD_PART even if none of any partition table
types (CONFIG_*_PARTITION) are enabled.
This will lead to the size growth in SPL/TPL code since disk/part.c
will be compiled in any way.
We will change disk/Kconfig later so that CONFIG_PARTITIONS is only
enabled when, at least, one of CONFIG_*_PARTITION is enabled.

To make the build work (in particular, "part" command) correctly,
a few functions should be defined as void functions in case of
!CONFIG_PARTITIONS.

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