target/xtensa: fix ICACHE/DCACHE options detection

Configuration overlay does not explicitly say whether there are ICACHE
and DCACHE in the core. Current code uses XCHAL_[ID]CACHE_WAYS to detect
if corresponding cache option is enabled, but that's not correct: on
cores without cache these macros are defined as 1, not as 0.
Check XCHAL_[ID]CACHE_SIZE instead.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
