net: phy: marvell: add Marvell specific PHY loopback

Existing genphy_loopback() is not applicable for Marvell PHY. Besides
configuring bit-6 and bit-13 in Page 0 Register 0 (Copper Control
Register), it is also required to configure same bits  in Page 2
Register 21 (MAC Specific Control Register 2) according to speed of
the loopback is operating.

Tested working on Marvell88E1510 PHY for all speeds (1000/100/10Mbps).

FIXME: Based on trial and error test, it seem 1G need to have delay between
soft reset and loopback enablement.

Fixes: 014068dc

 ("net: phy: genphy_loopback: add link speed configuration")
Cc: <stable@vger.kernel.org> # 5.15.x
Signed-off-by: Mohammad Athari Bin Ismail <mohammad.athari.ismail@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>