Documentation: remove old sbc8260 board specific information

This file contains 8 yr. old board specific information that was for
the now gone ppc implementation, and it pre-dates widespread u-boot
support.  Any of the technical details of the board memory map would be
more appropriately captured in a dts if I revive it as powerpc anyway.

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