ARM: 7089/1: L2X0: add explicit cpu_relax() for busy wait loop

using cpu_relax in busy loops is a well-known idiom in the kernel.
It's more for documentation purposes than technically needed here.

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>