staging: vc04_services: clarify firmware dependency

The raspberrypi-firmware driver may be built as a loadable module,
which causes a link-time failure if the vc04_services driver is
built-in during compile-testing:

drivers/staging/vc04_services/vchiq.o: In function `vchiq_probe':
vchiq_connected.c:(.text.vchiq_probe+0x2c): undefined reference to `rpi_firmware_get'
drivers/staging/vc04_services/vchiq.o: In function `vchiq_platform_init':
vchiq_connected.c:(.text.vchiq_platform_init+0x1f0): undefined reference to `rpi_firmware_property'

This extends the dependency list to ensure the firmware is either
reachable, or completely disabled in case of compile-testing.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>