board_init.c: Always use memset()

We can make the code read more easily here by simply using memset()
always as when we don't have an optimized version of the function we
will still have a version of this function around anyhow.

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