arm: mvebu: Enable L2 cache on Armada XP

Until now, the L2 cache was never enabled again in U-Boot. To get
even better performance (bootup time), lets enable the L2 cache
in U-Boot. This code was taken from the Linux kernel.

A performance gain was measured on the DB-MV784MP-GP board by testing
with tftpboot and sata commands.

This patch also cleans up the L2 cache related code. And makes sure that
the L2 cache is only disabled once.

Please note that A38x still runs with L2 cache disabled. And needs
to be enabled for this SoC in a separate patch if needed or desired.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>