Document CONFIG_IMXRT_RPTUN: what the driver does at start and stop,
the memory map rules the board must follow (only the first 128 KB of
the backdoor window reaches the CM4, boot from the backdoor alias,
shared window mapped non-cacheable), the carveout resource the CM4
table must carry for the vrings and buffers, and the kick ordering the
CM4 firmware must honour.
Assisted-by: Claude:claude-fable-5-1
Signed-off-by: Lourens Naude <lourens@bearmetal.eu>
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