drivers: sh: Disable PM runtime for multi-platform sh73a0 with genpd

If the default PM domain using PM_CLK is used for PM runtime, the real PM
domain(s) cannot be registered from DT later.

Hence do not enable it when running a multi-platform kernel with genpd
support on an sh73a0. The R-Mobile PM domain driver will take care of
PM runtime management of the module clocks.

The default PM domain is still needed for:
  - platforms without genpd support,
  - the legacy (non-DT) case, where genpd may take over later, except
    for the C5 "always on" PM domain.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>