staging: bcm2835-v4l2: Add a build system for the module.

This is derived from the downstream tree's build system, but with just
a single Kconfig option.

For now the driver only builds on 32-bit arm -- the aarch64 build
breaks due to the driver using arm-specific cache flushing functions.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>