PM / Domains: Allow holes in genpd_data.domains array

In platforms such as Rockchip's, the array of domains isn't always
filled without holes, as which domains are present depend on the
particular SoC revision.

By allowing holes to be in the array, such SoCs can still use a single
set of constants to index the array of power domains.

Fixes: 0159ec67 (PM / Domains: Verify the PM domain is present when adding a provider)
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>