net: ethernet: oa_tc6: implement register read operation

net: ethernet: oa_tc6: implement register read operation

Implement register read operation according to the control communication
specified in the OPEN Alliance 10BASE-T1x MACPHY Serial Interface
document. Control read commands are used by the SPI host to read
registers within the MAC-PHY. Each control read commands are composed of
a 32 bits control command header.

The MAC-PHY ignores all data from the SPI host following the control
header for the remainder of the control read command. Control read
commands can read either a single register or multiple consecutive
registers. When multiple consecutive registers are read, the address is
automatically post-incremented by the MAC-PHY. Reading any unimplemented
or undefined registers shall return zero.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Parthiban Veerasooran <Parthiban.Veerasooran@microchip.com>
Link: https://patch.msgid.link/20240909082514.262942-4-Parthiban.Veerasooran@microchip.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>