dt-bindings: document a note about power domain subdomains

This patch adds a note on defining subdomains to generic PM domain
binding documentation to let power domain providers use common approach
for defining power domain hierarchy.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Kukjin Kim <kgene@kernel.org>