dt-bindings: net: realtek,rtl82xx: Document realtek,*-ssc-enable property

dt-bindings: net: realtek,rtl82xx: Document realtek,*-ssc-enable property

Document support for spread spectrum clocking (SSC) on RTL8211F(D)(I)-CG,
RTL8211FS(I)(-VS)-CG, RTL8211FG(I)(-VS)-CG PHYs. Introduce DT properties
'realtek,clkout-ssc-enable', 'realtek,rxc-ssc-enable' and
'realtek,sysclk-ssc-enable' which control CLKOUT, RXC and SYSCLK
SSC spread spectrum clocking enablement on these signals. These
clock are not exposed via the clock API, therefore assigned-clock-sscs
property does not apply.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Signed-off-by: Marek Vasut <marek.vasut@mailbox.org>
Link: https://patch.msgid.link/20260405233008.148974-2-marek.vasut@mailbox.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>