drm/vc4: Enable selection in Kconfig on any 32-bit BCM platform.

With the Cygnus port, we needed to add at least "|| ARCH_BCM_CYGNUS"
to let the module get built on a cygnus-only kernel.  However, I
anticipate having a port for Kona soon, so just present the module on
all of BCM.

v2: Keep allowing selection with ARCH_BCM2835, since ARCH_BCM doesn't
    exist on arm64.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch> (v1)
Acked-by: Scott Branden <scott.branden@broadcom.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170509181539.30278-1-eric@anholt.net