riscv: Add the Allwinner SoC family Kconfig option

riscv: Add the Allwinner SoC family Kconfig option

Allwinner manufactures the sunxi family of application processors. This
includes the "sun8i" series of ARMv7 SoCs, the "sun50i" series of ARMv8
SoCs, and now the "sun20i" series of 64-bit RISC-V SoCs.

The first SoC in the sun20i series is D1, containing a single T-HEAD
C906 core. D1s is a low-pin-count variant of D1 with co-packaged DRAM.

Most peripherals are shared across the entire chip family. In fact, the
ARMv7 T113 SoC is pin-compatible and almost entirely register-compatible
with the D1s.

This means many existing device drivers can be reused. To facilitate
this reuse, name the symbol ARCH_SUNXI, since that is what the existing
drivers have as their dependency.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Link: https://lore.kernel.org/r/20230126045738.47903-11-samuel@sholland.org
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>