sbc8548: replace README with completely new document

The previous README.sbc8548 was pretty much content-free. Replace
it with something that actually gives the end user some relevant
hardware details, and also lists the u-boot configuration choices.

Also in the cosmetic department, fix the bogus line in the Makefile
that was carried over from the SBC8560 Makefile, and the typo in
the sbc8548.c copyright.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>