disk: enable function prototypes in part.h for SPL/TPL

Since CONFIG_[SPL|TPL]_PARTITIONS were introduced, part.h has not been
updated. Due to this, while the build won't fail, some functionality may
possibly break as some partition-related functions are nullified even
though some partition table types are enabled for SPL/TPL.

Fixes: commit 88ca8e26 ("disk: Add an option for partitions in SPL")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>