dt-bindings: Add common bindings for ARM and RISC-V idle states

The RISC-V CPU idle states will be described in under the
/cpus/idle-states DT node in the same way as ARM CPU idle
states.

This patch adds common bindings documentation for both ARM
and RISC-V idle states.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>