net: phy: broadcom: Add LPI counter

Add the ability to read the PHY maintained LPI counter which is in the
Clause 45 vendor space, device address 7, offset 0x803F. The counter is
cleared on read.

Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20230531231729.1873932-1-florian.fainelli@broadcom.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>