ARM: 6859/1: Add writethrough dcache support for ARM926EJS processor

The ARM kernel supports writethrough data cache via the
CONFIG_CPU_DCACHE_WRITETHROUGH option.  However, that
functionality wasn't implemented in the arch/arm/boot/compressed
code.  It is now necessary due to a new ARM926EJS processor
that has an issue with writeback data cache.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>