riscv: dts: thead: change TH1520 SPI node to use clock controller

riscv: dts: thead: change TH1520 SPI node to use clock controller

Change the clock property in the TH1520 SPI controller node to a clock
provided by AP_SYS clock controller.

Remove spi_clk fixed clock reference from BeagleV Ahead and LPI4a dts.

Link: https://git.beagleboard.org/beaglev-ahead/beaglev-ahead/-/tree/main/docs
Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>