firmware: raspberrypi: Introduce devm_rpi_firmware_get()

It'll simplify the firmware handling for most consumers.

Suggested-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>