ARM: 7546/1: cache-l2x0: add an optional register to save/restore

Tested-and-Reviewed-by: Yehuda Yitschak <yehuday@marvell.com>
Tested-and-Reviewed-by: Lior Amsalem <alior@marvell.com>

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>