configure: Add the new "vc5" driver to the list, requiring a simulator.

configure: Add the new "vc5" driver to the list, requiring a simulator.

My intent is to develop the vc5 driver in-tree for some time to build the
CL generation and shader compiler code, and keep out-of-tree patches for
talking to an actual kernel driver until the kernel driver can be
stabilized on the hardware.

v2: Define a HAVE_BROADCOM_DRIVERS, like HAVE_INTEL or HAVE_AMD.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>