[ARM] Kirkwood: invalidate L2 cache before enabling it

I get random oopses on my Kirkwood board at startup when L2 cache is
enabled. FYI I'm using Marvell uboot version 3.4.16

Each boot produces the same oops, but anything that changes the kernel
size (even only changing initramfs) makes the oops different.

I noticed that nothing invalidates the L2 cache before enabling it,
doing so fixes my problem.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Nicolas Pitre <nico@marvell.com>