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 88ca8e2695 ("disk: Add an option for partitions in SPL")
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>

include/part.h

@@ -87,7 +87,7 @@ struct disk_part {
 };
 
 /* Misc _get_dev functions */
-#ifdef CONFIG_PARTITIONS
+#if CONFIG_IS_ENABLED(PARTITIONS)
 /**
  * blk_get_dev() - get a pointer to a block device given its type and number
  *
@@ -497,7 +497,7 @@ int layout_mbr_partitions(struct disk_partition *p, int count,
 
 #endif
 
-#ifdef CONFIG_PARTITIONS
+#if CONFIG_IS_ENABLED(PARTITIONS)
 /**
  * part_driver_get_count() - get partition driver count
  *