net: zynq_gem: Wait for SGMII PCS link in zynq_gem_init()

In our system using ZynqMP with an external SGMII PHY it's necessary
to wait for the PCS link and auto negotiation to finish before the xfer
starts. Otherwise the first packet(s) might get dropped, resulting in a
delay at the start of the ethernet transfers.

This is only done when the PHY link is already up, which is done in
phy_startup(). As waiting for the PHY link bits via pcsstatus does not
make much sense, when the link is not available in general (e.g. no
cable connected).

This patch adds the necessary code including a minimal delay of 1 ms
which fixes problems of dropped first packages.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Michal Simek <michal.simek@amd.com>
Cc: Katakam Harini <harini.katakam@amd.com>
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Sean Anderson <sean.anderson@seco.com>
Link: https://lore.kernel.org/r/20230125070908.1343256-1-sr@denx.de
Signed-off-by: Michal Simek <michal.simek@amd.com>

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