arm926ejs: remove noop flush_dcache_all function

arm926ejs: remove noop flush_dcache_all function

Commit 2f3427c added noop cache functions implementation for arm926ejs
to fix compilation of drivers depending on these functions (DaVinci
EMAC in particular).

Unfortunately, the bug was introduced: noop implementation calls
dcache_disable which calls flush_dcache_all which in turn calls
dcache_disable thus creating an infinite loop.

This patch removes noop implementation for flush_dcache_all, we already
have default one in arch/arm/lib/cache.c and it should be used instead.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Tested-by: Matthias Weisser <weisserm@arcor.de>