part: Add an accessor for struct disk_partition sys_ind

part: Add an accessor for struct disk_partition sys_ind

This field is only present when a CONFIG is set. To avoid annoying #ifdefs
in the source code, add an accessor. Update the only usage.

Note that the accessor is optional. It can be omitted if it is known that
the option is enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>