MAINTAINERS: add `dts/riscv` to the `RISCV arch` area of maintenance

This commit adds the `dts/riscv` directory to the `RISCV arch` area of
maintenance. Right now, RISC-V devicetrees are covered only by
platform-specific areas (Espressif, ITE). When other RISC-V platform
devicetrees are affected in a PR, it is left unassigned.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>