riscv: dts: thead: add BeagleV Ahead board device tree

The BeagleV Ahead single board computer uses the T-Head TH1520 SoC.
Add a minimal device tree to support basic uart/gpio/dmac drivers so
that a user can boot to a basic shell.

Link: https://beagleboard.org/beaglev-ahead


Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>