ARM: shmobile: defconfig: Refresh for v6.1-rc5

Refresh the defconfig for Renesas ARM systems:
  - Disable CONFIG_DRM_RCAR_USE_MIPI_DSI (defaults to yes since commit
    a830a156 ("drm: rcar-du: Fix Kconfig dependency between
    RCAR_DU and RCAR_MIPI_DSI"), but only used on R-Car V3U).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/4f7757bd700edff487df387ca40ffb1524d688a4.1672744302.git.geert+renesas@glider.be