ARM: 6294/1: etm: do a dummy read from OSSRR during initialization

The first read from ETM OS save and restore register after the power
down bit deassertion returns garbage.

Change-Id: Ia65620f364076175a2bb1b81d22f77da11f4dfa8
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Alexander Shishkin <virtuoso@slind.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>