ARM: dts: r8a7779, marzen: Fix DU clock names

"make dtbs_check" complains:

    arch/arm/boot/dts/r8a7779-marzen.dt.yaml: display@fff80000: clock-names:0: 'du.0' was expected

Change the first clock name to match the DT bindings.
This has no effect on actual operation, as the Display Unit driver in
Linux does not use the first clock name on R-Car H1, but just grabs the
first clock.

Fixes: 665d79aa ("ARM: shmobile: marzen: Add DU external pixel clock to DT")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://lore.kernel.org/r/9d5e1b371121883b3b3e10a3df43802a29c6a9da.1619699965.git.geert+renesas@glider.be