armv7: Use isb/dsb directly in start.S

Toolchains which do not directly support using "isb" and "dsb" directly
are no longer functionally supported in U-Boot. Furthermore, clang has
for a long time warned about using the alternate form that we were.
Update the code.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>