firmware: raspberrypi: Fix firmware calls with large buffers

Commit a1547e0b ("firmware: raspberrypi: Remove VLA usage")
moved away from VLA's to a fixed maximum size for mailbox data.
However, some mailbox calls use larger data buffers
than the maximum allowed in that change. This fix therefor
moves from using fixed buffers to kmalloc to ensure all sizes
are catered for.

There is some documentation, which is somewhat out of date,
on the mailbox calls here :
https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface



Fixes: a1547e0b ("firmware: raspberrypi: Remove VLA usage")

Signed-off-by: James Hughes <james.hughes@raspberrypi.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>