spi/pl022: use cpu_relax in the busy loop

This relaxes the cpu in the polling busy-wait loop.
Reported-by: Vitaly Wool <vitalywool@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/amba-pl022.c

@@ -1424,6 +1424,7 @@ static void do_polling_transfer(struct pl022 *pl022)
 				message->state = STATE_ERROR;
 				goto out;
 			}
+			cpu_relax();
 		}
 
 		/* Update total byte transferred */