overlays: Add real parameters to the rpi-poe overlay

As a result of being loaded by the POE HAT EEPROM, the rpi-poe overlay
doesn't expose parameters in the usual way; instead it adds them to
the base Device Tree, and the user is expected to use "dtparam=..."
to access them.

To make the documentation correct and to protect users who load the
overlay explicitly, expecting to be able to use the parameters, add
real parameters to the overlay as well.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>