sh: avoid to flush all cache in sys_cacheflush

Calling sys_cacheflush with ICACHE we can direclty flush
the icache without invoking the flush_cache_all function.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>