dts: add riscv include prefix link

The Allwinner D1/D1s SoCs (with a RISC-V core) use an (almost?) identical
die as their R528/T113-s siblings with ARM Cortex-A7 cores.

To allow sharing the basic SoC .dtsi files across those two
architectures as well, introduce a symlink to the RISC-V DT directory.

Signed-off-by: Andre Przywara <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Acked-by: Palmer Dabbelt <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jernej Skrabec <[e-mail]>