Make set_performance_state() callback optional in order to remove the
need from power domain drivers to implement a dummy callback. If callback
isn't implemented by a GENPD driver, then the performance state is passed
to the parent domain.

Tested-by: Peter Geis <redacted>
Tested-by: Nicolas Chauvet <redacted>
Tested-by: Matt Merhar <redacted>
[tested on NVIDIA Tegra20/30/124 SoCs]
Suggested-by: Ulf Hansson <redacted>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
 drivers/base/power/domain.c | 33 ++++++++++++++++++---------------
 1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/drivers/base/power/domain.c b/drivers/base/power/domain.c
index 9a14eedacb92..0bd0cdc30393 100644
--- a/drivers/base/power/domain.c
+++ b/drivers/base/power/domain.c

@@ -297,6 +297,18 @@ static int _genpd_reeval_performance_state(struct generic_pm_domain *genpd,
 	return state;
 }
 
+static int _genpd_xlate_performance_state(struct generic_pm_domain *src_genpd,
+					  struct generic_pm_domain *dst_genpd,
+					  unsigned int pstate)