ARC: Cache: Move PAE exists check into slc_upper_region_init()

Move check for PAE existence into slc_upper_region_init()
instead of its caller as more appropriate place.

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