efi_loader: Always flush in cache line size granularity

The cache line flush helpers only work properly when they get aligned
start and end addresses. Round our flush range to cache line size. It's
safe because we're guaranteed to flush within a single page which has the
same cache attributes.

Reported-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Tested-by: Andreas Färber <afaerber@suse.de>