dt-bindings: net: thead,th1520-gmac: Describe APB interface clock

dt-bindings: net: thead,th1520-gmac: Describe APB interface clock

Besides ones for GMAC core and peripheral registers, the TH1520 GMAC
requires one more clock for configuring APB glue registers. Describe
it in the binding.

Fixes: f920ce04c399 ("dt-bindings: net: Add T-HEAD dwmac support")
Signed-off-by: Yao Zi <ziyao@disroot.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Drew Fustini <fustini@kernel.org>
Link: https://patch.msgid.link/20250808093655.48074-3-ziyao@disroot.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>