net: phy: mscc: Enable RMII clock output for VSC8541 PHY

net: phy: mscc: Enable RMII clock output for VSC8541 PHY

Set RMII reference clock output to enabled (1) by default for VSC8541
PHY in RMII mode. The RMII specification requires a 50MHz reference
clock, and many board designs expect the PHY to provide this clock to
the MAC controller.

Previously, the driver defaulted rmii_clk_out to 0 (disabled) for all
interface modes, which caused the PHY to not output the required 50MHz
clock. This resulted in MAC-PHY communication failures and prevented
network operations like DHCP from working on RMII-configured boards.

This change alligns with the hardware power-up default behavior and
aligns with both the generic PHY driver and Linux MSCC PHY driver
implementations.

Signed-off-by: Pranav Tilak <pranav.vinaytilak@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20260129081054.1703479-1-pranav.vinaytilak@amd.com