[ARM] 5293/1: ep93xx: add defines for external chipselects

[ARM] 5293/1: ep93xx: add defines for external chipselects

This patch adds defines for the external chipselect physical base
addresses available with the EP93xx. These are meant to be used
in the platform init code.

In addition, documentation about the synchronous/asynchronous boot
modes for the EP93xx and a reference to errata about issues with
synchronous booting has been added.

Signed-off-by: <H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>