package/rpi-firmware: fix unmet dependencies

Currently, vcdbg is only supported in 32-bit mode. Furthermore, vcdbg
needs rpi-userland, which we currently only support in 32-bit mode.

Add a dependency on BR2_arm.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>