ARC: Cache: Implement [i,d]cache_enabled() as separate functions

Implement icache_enabled() and dcache_enabled() as separate functions
which can be used with "inline" attribute. This is a preparation to
make them always_inline.

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