ARM: shmobile: defconfig: Refresh for v6.16-rc2

Refresh the defconfig for Renesas ARM systems:
  - Disable CONFIG_DWMAC_RENESAS_GBETH, which defaults to yes since
    commit 461f6529 ("net: stmmac: Add DWMAC glue layer for
    Renesas GBETH"), but is only used on Renesas ARM64 SoCs.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/f04a2b0e2c9128b4fa644850270b55493358be39.1750074375.git.geert+renesas@glider.be