The imx6sl-evk board has a LAN8720A ethernet phy supported by SMSC_PHY.
Add this driver to the default imx config since the device is present on
one of the evaluation boards.

This used to work mostly fine with the generic phy driver until
commit 0878fff1 ("net: phy: Do not perform software reset for
Generic PHY"). The fact that soft reset is no longer performed
apparently causes RX to sometimes failes which can cause netboot to
timeout on DHCP. This is eventually retried and it works after link
up/down but can takes 90 seconds to reach the login prompt.

This was generated with "make savedefconfig" and it includes a few
additional minor cleanups.
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>