remoteproc: stm32: demote warning about optional property absence

The mainline device trees lack the "st,syscfg-pdds", so probing the
driver always throws the warning. As the property is optional per binding
and the driver can deal with its absence, demote the warning to info log
level.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Link: https://lore.kernel.org/r/20200224172519.15315-1-a.fatoum@pengutronix.de


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>