can: sun4i_can: Only show Kconfig if ARCH_SUNXI is set

When adding the RISCV option I didn't gate it behind ARCH_SUNXI.
As a result this option shows up with Allwinner support isn't enabled.
Fix that by requiring ARCH_SUNXI to be set if RISCV is set.

Fixes: 8abb9525 ("can: sun4i_can: Add support for the Allwinner D1")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Closes: https://lore.kernel.org/linux-sunxi/CAMuHMdV2m54UAH0X2dG7stEg=grFihrdsz4+o7=_DpBMhjTbkw@mail.gmail.com/


Signed-off-by: John Watts <contact@jookia.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/all/20230905231342.2042759-2-contact@jookia.org


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>