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>