firmware/psci: Add debugfs support to ease debugging

To ease debugging of PSCI supported features, add debugfs file called
'psci' describing PSCI and SMC CC versions, enabled features and
options.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20220926110758.666922-1-dmitry.baryshkov@linaro.org

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