smc911x: split out useful defines/functions into local header

The smc911x driver has a lot of useful defines/functions which can be used
by pieces of code (such as example eeprom programmers).  Rather than
forcing each place to duplicate these defines/functions, split them out
of the smdc911x driver into a local header.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Ben Warren <biggerbadderben@gmail.com>
CC: Sascha Hauer <s.hauer@pengutronix.de>
CC: Guennadi Liakhovetski <lg@denx.de>
CC: Magnus Lilja <lilja.magnus@gmail.com>
CC: Ben Warren <biggerbadderben@gmail.com>