dt-bindings: riscv: Add BeagleV Ahead board compatibles

Document the compatible strings for the BeagleV Ahead board which uses
the T-Head TH1520 SoC.

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


Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
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>