arm, arm926ejs: Enable icache only if CONFIG_SYS_ICACHE_OFF is not defined

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@ti.com>
Acked-by: Heiko Schocher <hs@denx.de>
Tested-by: Heiko Schocher <hs@denx.de>