clk: renesas: mstp: Drop check for CONFIG_PM_GENERIC_DOMAINS_OF

As of commit 71d076ce ("ARM: shmobile: Enable PM and
PM_GENERIC_DOMAINS for SoCs with PM Domains"),
CONFIG_PM_GENERIC_DOMAINS_OF is always enabled for SoCs with MSTP
clocks.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>