net: phy: spi_ks8995: add support for resetting switch using GPIO

When using device tree it is no more possible to reset the PHY at board
level. Furthermore, doing in the driver allows to power down the switch
when it is not used any more.

The patch introduces a new optional property "reset-gpios" denoting an
appropriate GPIO handle, e.g.:

reset-gpios = <&gpio0 46 GPIO_ACTIVE_LOW>

Signed-off-by: Helmut Buchsbaum <helmut.buchsbaum@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>