pmdomain: Make genpd_get_from_provider() public

Unlike the corresponding function in the clock subsystem
(of_clk_get_from_provider()), genpd_get_from_provider() is private, and
thus cannot be used by PM Domain drivers.

Make it public, so it be used by the R-Car X5H Module Controller driver.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>