riscv: defconfig: Disable Renesas SoC support

Follow-up to commit e36ddf32 ("riscv: defconfig: Disable RZ/Five
peripheral support") in v6.12-rc1:

  - Disable ARCH_RENESAS, too, as currently RZ/Five is the sole Renesas
    RISC-V SoC,
  - Drop no longer needed explicit disable of USB_XHCI_RCAR, which
    depends on ARCH_RENESAS.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/e8a2fb273c8c68bd6d526b924b4212f397195b28.1738764211.git.geert+renesas@glider.be


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>