net: mdio: add a forward declaration for reset_control to mdio.h

This header refers to struct reset_control but doesn't include any reset
header. The structure definition is probably somehow indirectly pulled in
since no warnings are reported but for the sake of correctness add the
forward declaration for struct reset_control.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>