arm64: reset: flush D-Cache before it is disabled

In the commit 573a712b patch "arm64:
reset: disable cache and MMU for safety", it disables D-Cache and MMU
for safety, but in some cases, for example the code is loaded into memory
by hardware debugger, we need to flush D-Cache before disable it in
order to make sure the data is coherent in the system, otherwise it
will report "Synchronous Abort" when D-Cache is disabled.

Signed-off-by: Jiafei Pan <Jiafei.Pan@nxp.com>