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>

MAINTAINERS.yml

@@ -2127,6 +2127,7 @@ RISCV arch:
     - arch/riscv/
     - boards/riscv/
     - dts/bindings/riscv/
+    - dts/riscv/
     - include/zephyr/arch/riscv/
     - soc/riscv/
     - tests/arch/riscv/