firmware: raspberrypi: Keep count of all consumers

[ Upstream commit 1e7c57355a3bc617fc220234889e49fe722a6305 ]

When unbinding the firmware device we need to make sure it has no
consumers left. Otherwise we'd leave them with a firmware handle
pointing at freed memory.

Keep a reference count of all consumers and introduce rpi_firmware_put()
which will permit automatically decrease the reference count upon
unbinding consumer drivers.

Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Stable-dep-of: 5bca3688bdbc ("Input: raspberrypi-ts - fix refcount leak in rpi_ts_probe")
Signed-off-by: Sasha Levin <sashal@kernel.org>