realtek: add RTL8231 chip detection

When initialising the driver, check if the RTL8231 chip is actually
present at the specified address. If the READY_CODE value does not match
the expected value, return -ENXIO to fail probing.

This should help users to figure out which address an RTL8231 is
configured to use, if measuring pull-up/-down resistors is not an
option.

On an unsuccesful probe, the driver will log:
    [    0.795364] Probing RTL8231 GPIOs
    [    0.798978] rtl8231_init called, MDIO bus ID: 30
    [    0.804194] rtl8231-gpio rtl8231-gpio: no device found at bus address 30

When a device is found, only the first two lines will be logged:
    [    0.453698] Probing RTL8231 GPIOs
    [    0.457312] rtl8231_init called, MDIO bus ID: 31

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Tested-by: Stijn Tintel <stijn@linux-ipv6.be>