drivers: eth_smsc91x: Add driver for SMSC91C111 aka LAN91C111 chip

Arm fvp_baser_aemv8r and fvp_base_revc_2xaemv8a boards are using
SMSC91C111 as their ethernet adapters.

Portions of the codes are based on FreeBSD code from its
'src/sys/dev/smc/if_smc.c' and 'src/sys/dev/smc/if_smcreg.h'.

This driver has two parts, one is the ethernet controller driver, which
is MAC layer driver. The other is the MDIO driver, which is the PHY
layer driver. Both of them are in the same source file due to that they
need to share the same reading and writing register functions and
the smsc object.

The mdio driver is needed by the existing 'phy_mii' driver, which is
a driver for the generic MII-compliant PHY.

This driver was developed under the fvp_base_revc_2xaemv8a target and
has been tested on the fvp_baser_aemv8r target.

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>