dt-bindings: Fix phandle-array issues in the idle-states bindings

As per 39bd2b6a ("dt-bindings: Improve phandle-array schemas"), the
phandle-array bindings have been disambiguated.  This fixes the new
RISC-V idle-states bindings to comply with the schema.

Fixes: 1bd524f7 ("dt-bindings: Add common bindings for ARM and RISC-V idle states")
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>