Add missing L3 cache flush functionality which absence prevents
Linux kernel from normal boot in case the L3 cache is enabled
by ATF.
The L3 cache is named the "last level" cache in order to keep
the terminology similar to the ATF code.
This cache should not be disabled by u-boot since the Linux
kernel cannot activate it, so it is activates at ATF stage.
However the cache flush is required for preventing data corruption
after disabling the MMU and the data cache before passing control
to the loaded Linux image.

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Neta Zur Hershkovits <neta@marvell.com>
Cc: Omri Itach <omrii@marvell.com>
Cc: Igal Liberman <igall@marvell.com>
Cc: Haim Boot <hayim@marvell.com>
Cc: Hanna Hawa <hannah@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>