ARM: shmobile: defconfig: Refresh for v5.9-rc1

Refresh the defconfig for Renesas ARM systems:
  - Drop CONFIG_BACKLIGHT_GENERIC=n (removed in commit 7ecdea4a
    ("backlight: generic_bl: Remove this driver as it is unused")).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200819101125.12113-1-geert+renesas@glider.be