riscv: dts: thead: Add APB clocks for TH1520 GMACs

Describe perisys-apb4-hclk as the APB clock for TH1520 SoC, which is
essential for accessing GMAC glue registers.

Fixes: 7e756671 ("riscv: dts: thead: Add TH1520 ethernet nodes")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Reviewed-by: Drew Fustini <fustini@kernel.org>
Tested-by: Drew Fustini <fustini@kernel.org>
Link: https://patch.msgid.link/20250808093655.48074-5-ziyao@disroot.org


Signed-off-by: Paolo Abeni <pabeni@redhat.com>