armv7m: cache: add flush & invalidate all dcache

Add functionality to flush & invalidate all the dcache using the
prototype declared in common header file.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
[trini: Add dummy functions for the not-enabled case]
Signed-off-by: Tom Rini <trini@konsulko.com>