kernel: qca-ssdk: set OF node for the SFP PHY

kernel: qca-ssdk: set OF node for the SFP PHY

Currently, SSDK is creating a fake SFP PHY which has no OF node populated,
thus making it impossible to pass it to NSS-DP so port can actually work.

We eliminated QCA-s connecting of the PHY by manually creating a string
and then matching by name and instead only support passing the PHY as
phandle via phy-handle.

So, lets just use the switch port node to which the SFP is connected to
anyway and set it as the PHY device OF node so we can pass it to NSS-DP.

Link: https://github.com/openwrt/openwrt/pull/15840
Signed-off-by: Robert Marko <robimarko@gmail.com>