mips: mt76xx: Implement new d-cache fix in last_stage_init()

With commit 06985289d452 ("watchdog: Implement generic watchdog_reset()
version") the init sequence has changed in arch_misc_init(), resulting
in a re-appearance of the d-cache issue on MT7688 boards (e.g. gardena).
When this happens, the first (or sometimes later ones as well) TFTP
command hangs and does not complete correctly. This leads to the
assumption that the d-cache is not in a clean state once the ethernet
driver is called (d-cache is used here for the buffers). The old work-
around with the cache flush somehow does not work any more now with
the new code change.

Unfortunately adding CONFIG_SYS_MALLOC_CLEAR_ON_INIT also did not fix
this issue. With v2019.07-rc3 it shows again. The time of accessing
the data seems to be very important here. It needs to be "very late"
in the boot process.

Testing has shown, that copying a 64KiB area in DDR at a very late
bootup time, directly before calling into the prompt, fixes this issue.
Flushing of the complete d-cache does not seem to necessary, as this
copy alone seems to fix this problem.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

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