clk: renesas: cpg-mssr: Set GENPD_FLAG_ALWAYS_ON for clock domain

The CPG/MSSR 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.

Note that this only affects RZ/A2 SoCs.  On R-Car Gen2 and Gen3 SoCs,
the R-Car SYSC driver handles Clock Domain creation, and offloads only
device attachment/detachment to the CPG/MSSR driver.

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>