ARC: cleanup instruction cache initialization

As of today during the Zephyr start we
 - invalidate I$
 - disable I$
 - enable I$

Given that we don't need to have I$ disabled during any
initialization period and ARC processors have caches enabled
after reset the I$ disabling/enabling is excessive, so we can
drop it.

By that we also aligh the I$ initialization on ARC with other
projects like U-boot and Linux kernel.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>