overlays: rpi-poe-plus: Improve the cooling levels

The PoE HAT cooling levels are not well suited for the PoE+ HAT - the
fan fails to come on until the temperature reaches the third trip point
(50C).

Give the rpi-poe-plus overlay a different set of cooling levels -
0 32 64 128 255, as suggested by @chris-kai-in.

See: https://github.com/raspberrypi/firmware/issues/1607



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