media: raspberrypi: VIDEO_RASPBERRYPI_PISP_BE should depend on ARCH_BCM2835

Currently, the Raspberry Pi PiSP Backend (BE) ISP is only present on the
Broadcom BCM2712-based Raspberry Pi 5.  Hence add a dependency on
ARCH_BCM2835, to prevent asking the user about this driver when
configuring a kernel without Broadcom BCM2835 family support.  The
dependency can be relaxed if/when the encoder appears on other SoC
families.

Fixes: 12187bd5 ("media: raspberrypi: Add support for PiSP BE")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: FLorian Fainelli <florian.fainelli@broadcom.com>
Acked-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>