phy: xilinx: phy-zynqmp: Fix SGMII linkup failure on resume

[ Upstream commit 5af9b304bc6010723c02f74de0bfd24ff19b1a10 ]

On a few Kria KR260 Robotics Starter Kit the PS-GEM SGMII linkup is not
happening after the resume. This is because serdes registers are reset
when FPD is off (in suspend state) and needs to be reprogrammed in the
resume path with the same default initialization as done in the first
stage bootloader psu_init routine.

To address the failure introduce a set of serdes registers to be saved in
the suspend path and then restore it on resume.

Fixes: 4a33bea0

 ("phy: zynqmp: Add PHY driver for the Xilinx ZynqMP Gigabit Transceiver")
Signed-off-by: Piyush Mehta <piyush.mehta@amd.com>
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Link: https://lore.kernel.org/r/1722837547-2578381-1-git-send-email-radhey.shyam.pandey@amd.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>