ARM: socfpga: Clear PL310 early in SPL

On SoCFPGA Gen5 systems, it can rarely happen that a reboot from Linux
will result in stale data in PL310 L2 cache controller. Even if the L2
cache controller is disabled via the CTRL register CTRL_EN bit, those
data can interfere with operation of devices using DMA, like e.g. the
DWMMC controller. This can in turn cause e.g. SPL to fail reading data
from SD/MMC.

The obvious solution here would be to fully reset the L2 cache controller
via the reset manager MPUMODRST L2 bit, however this causes bus hang even
if executed entirely from L1 I-cache to avoid generating any bus traffic
through the L2 cache controller.

This patch thus configures and enables the L2 cache controller very early
in the SPL boot process, clears the L2 cache and disables the L2 cache
controller again.

The reason for doing it in SPL is because we need to avoid accessing any
of the potentially stale data in the L2 cache, and we are certain any of
the stale data will be below the OCRAM address range. To further reduce
bus traffic during the L2 cache invalidation, we enable L1 I-cache and
run the invalidation code entirely out of the L1 I-cache.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Dalon Westergreen <dwesterg@gmail.com>
Cc: Dinh Nguyen <dinguyen@kernel.org>

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