arm: Add support for Banana Pi board

The Banana Pi is a cheap ARMv7 board with a dual-core Cortex-A7, thus
with virtualization support. Upstream U-boot and kernel work fine -
ideal conditions. We just lack some IOMMU on that board, but it remains
handy for testing purposes.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>