clk: renesas: cpg-mssr: Use genpd of_node instead of local copy

Since commit 6a0ae73d ("PM / Domain: Add support to parse
domain's OPP table"), of_genpd_add_provider_simple() fills in
the dev.of_node field in the generic_pm_domain structure.

Hence cpg_mssr_is_pm_clk() can use that instead of its own copy in the
driver-private cpg_mssr_clk_domain structure.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>