arc: check caches existence before use

Some cache operations ({i|d}cache_{enable|disable|status} or
flush_dcache_all) are built and used even if CONFIG_SYS_{I|D}CACHE_OFF
is set.

This is required for force disable of caches on early boot.
What if something was executed before U-boot and enabled caches
(low-level bootloaders, previously run kernel etc.)?

But if CPU doesn't really have caches any attempt to access
cache-related AUX registers triggers instruction error exception.

So for convenience we'll try to avoid exceptions by checking if CPU
actually has caches (we check separately data and instruction cache
existence) at all.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Igor Guryanov <guryanov@synopsys.com>