firmware/psci: Print a warning if PSCI doesn't accept PC mode

The function psci_pd_try_set_osi_mode() will print an error if enabling
OSI mode fails. To ease debugging PSCI issues print corresponding
message if switching to PC mode fails too.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20220926110249.666813-1-dmitry.baryshkov@linaro.org

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>