cmd: part: Cover both part type entries with GUID ifdef

The 'part type' subcommand depends on GUID partition table support.
The help text exposes one of two 'part type' subcommand entries,
hide both in case GUID partition table support is not enabled to
avoid confusing users.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>