ARM: 8023/1: remove remnants of the static DMA mapping

It looks like the static mapping area for DMA was replaced by dynamic
allocation into the vmalloc area by commit e9da6e99

 but the
information in Documentation/arm/memory.txt was not removed accordingly.

CONSISTENT_END in arch/arm/include/asm/memory.h has no more users and
can be removed as well.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>