ARM: UniPhier: disable L2 cache by lowlevel_init of U-Boot proper

The L2 cache is used as a temporary SRAM on SPL.
Now the secondary CPUs store the necessary code for jumping to
Linux on their L1 I-caches.  So, the L2 cache can be disabled
much earlier, at the very entry of U-Boot proper (lowlevel_init).
This makes the boot sequence clearer.
Also, as the L1 cache has been disabled by the start.S,
enable_caches() does not need to do it again.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>