net: ethernet: stmmac: dwmac-rk: Make the clk_phy could be used for external phy

For external phy, clk_phy should be optional, and some external phy
need the clock input from clk_phy. This patch adds support for setting
clk_phy for external phy.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: Chaoyi Chen <chaoyi.chen@rock-chips.com>
Link: https://patch.msgid.link/20250815023515.114-1-kernel@airkyi.com


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