ARM: dts: meson8b: add the APB bus

Various peripherals (Mali GPU, NAND controller, VPU; etc.) are located
in the APB bus. Describe this bus so we can add devices to it.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>