clk: renesas: mstp: Set GENPD_FLAG_ALWAYS_ON for clock domain

The CPG/MSTP Clock Domain driver does not implement the
generic_pm_domain.power_{on,off}() callbacks, as the domain itself
cannot be powered down.  Hence the domain should be marked as always-on
by setting the GENPD_FLAG_ALWAYS_ON flag, to prevent the core PM Domain
code from considering it for power-off, and doing unnessary processing.

This also gets rid of a boot warning when the Clock Domain contains an
IRQ-safe device, e.g. on RZ/A1:

    sh_mtu2 fcff0000.timer: PM domain cpg_clocks will not be powered off
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>

drivers/clk/renesas/clk-mstp.c

@@ -334,7 +334,8 @@ void __init cpg_mstp_add_clk_domain(struct device_node *np)
 		return;
 
 	pd->name = np->name;
-	pd->flags = GENPD_FLAG_PM_CLK | GENPD_FLAG_ACTIVE_WAKEUP;
+	pd->flags = GENPD_FLAG_PM_CLK | GENPD_FLAG_ALWAYS_ON |
+		    GENPD_FLAG_ACTIVE_WAKEUP;
 	pd->attach_dev = cpg_mstp_attach_dev;
 	pd->detach_dev = cpg_mstp_detach_dev;
 	pm_genpd_init(pd, &pm_domain_always_on_gov, false);