ARC: Cache: Get rid of [slc,pae,icache,dcache]_exists global variables

There is a problem with current implementation if we start U-Boot
from ROM, as we use global variables before ther initialization,
so these variables get overwritten when we copy .data section
from ROM.

Instead we'll use icache_exists(), dcache_exists(), slc_exists(), pae_exists()
functions which directly check BCRs every time.

In U-Boot case ops are used only during self-relocation and DMA
so we shouldn't be hit by noticeable performance degradation.

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