usb: net: introduce support for Moschip USB ethernet

introduce an 'mcs7830' driver for Moschip MCS7830 based (7730/7830/7832)
USB 2.0 Ethernet Devices

see "MCS7830 -- USB 2.0 to 10/100M Fast Ethernet Controller" at
http://www.asix.com.tw/products.php?op=pItemdetail&PItemID=109;74;109



the driver was implemented based on the U-Boot Asix driver with
additional information gathered from the Moschip Linux driver,
development was done on "Delock 61147" and "Logilink UA0025C" dongles

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Acked-by: Marek Vasut <marex@denx.de>