hw/arm/mps2-tz: Allow board to specify a boot RAM size

Currently we model the ITCM in the AN547's RAMInfo list. This is incorrect
because this RAM is really a part of the SSE-300. We can't just delete
it from the RAMInfo list, though, because this would make boot_ram_size()
assert because it wouldn't be able to find an entry in the list covering
guest address 0.

Allow a board to specify a boot RAM size manually if it doesn't have
any RAM itself at address 0 and is relying on the SSE for that, and
set the correct value for the AN547. The other boards can continue
to use the "look it up from the RAMInfo list" logic.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210510190844.17799-6-peter.maydell@linaro.org