ARM: 5998/1: ep93xx: added chip revision reading function

Added a new function: ep93xx_chip_revision() which reads chip revision
from the sysconfig register.

Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>