net: phy: micrel: add option to keep the preamble before sfd for KSZ9131

net: phy: micrel: add option to keep the preamble before sfd for KSZ9131

If the PHY_F_KEEP_PREAMBLE_BEFORE_SFD flag is set in the
phy_device::dev_flags field, the preamble will be kept before the start
frame delimiter (SFD) on the KSZ9131 PHY. This flag is not officially
documented by Micrel. However, information provided by NXP and Micrel
indicates that this flag ensures the PHY sends the full preamble instead
of removing it. The full discussion can be found on the NXP forum:
https://community.nxp.com/t5/i-MX-Processors/iMX8MP-eqos-not-working-for-10base-t/m-p/2151032

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260120203905.23805-3-eichest@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>