net: phy: micrel: Add loopback support for ksz9131

ksz9131 is configured for local loopback in a similar fashion as the
ksz9031, with a need for full-duplex operation, but with some extra
steps to take as specified in section 4.13.1 :

1. Configure the following registers:
- MMD 1C, Register 15 = EEEE
- MMD 1C, Register 16 = EEEE
- MMD 1C, Register 18 = EEEE
- MMD 1C, Register 1B = EEEE

These 4 registers are marked as "Reserved" in the register map.

When setting loopback up without configuring these 4 registers, the PHY
appears to shut its RXC down, which can trigger failures on MACs that
require it, such as stmmac.

The datasheet does not specify to which state the registers must be
reset when disabling loopback, so let's restore them to their measured
initial values.

This was discovered when trying to use stmmac selftests on imx8mp with a
ksz9131 connected in RGMII.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260728075222.956780-1-maxime.chevallier@bootlin.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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