firmware/psci: demote suspend-mode warning to info level

On some Qualcomm platforms, like SC8280XP, the attempt to set PC mode
during boot fails with PSCI_RET_DENIED and since commit 998fcd00
("firmware/psci: Print a warning if PSCI doesn't accept PC mode") this
is now logged at warning level:

	psci: failed to set PC mode: -3

As there is nothing users can do about the firmware behaving this way,
demote the warning to info level and clearly mark it as a firmware bug:

	psci: [Firmware Bug]: failed to set PC mode: -3

Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>