hw/arm/mps2-tz: Get armv7m_load_kernel() size argument from RAMInfo

hw/arm/mps2-tz: Get armv7m_load_kernel() size argument from RAMInfo

The armv7m_load_kernel() function takes a mem_size argument which it
expects to be the size of the memory region at guest address 0.  (It
uses this argument only as a limit on how large a raw image file it
can load at address zero).

Instead of hardcoding this value, find the RAMInfo corresponding to
the 0 address and extract its size.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Message-id: 20210215115138[email protected]