ARM: shmobile: defconfig: Refresh for v5.12-rc2

Refresh the defconfig for Renesas ARM systems:
  - Drop CONFIG_ENABLE_MUST_CHECK=n (removed in commit 19679394
    ("Compiler Attributes: remove CONFIG_ENABLE_MUST_CHECK")).

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20210310103213.2529983-1-geert+renesas@glider.be