firmware: raspberrypi: Introduce rpi_firmware_find_node()

A significant number of RaspberryPi drivers using the firmware don't
have a phandle to it, so end up scanning the device tree to find a node
with the firmware compatible.

That code is duplicated everywhere, so let's introduce a helper instead.

Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220815-rpi-fix-4k-60-v5-1-fe9e7ac8b111@cerno.tech


Signed-off-by: Maxime Ripard <maxime@cerno.tech>