Document bindings for the T-Head TH1520 AP sub-system clock controller.

Link: https://openbeagle.org/beaglev-ahead/beaglev-ahead/-/blob/main/docs/TH1520%20System%20User%20Manual.pdfCo-developed-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>
Link: https://lore.kernel.org/r/20240623-th1520-clk-v2-1-ad8d6432d9fb@tenstorrent.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>