added 16/24 bit mixed address space

[thomas-haller]

In a project of our company we had the following problems:
-small mode was not usable as the (used) flash area was larger than 32kB
-medium mode made to many problems in decompilation because of the zero extension of the stack pointer.
Our solution was to create a mixed mode. Similar to an harvard architecture we specified a 16 bit address bus "RAM" for all data accesses and a 24 bit address bus "ROM" for all code fetching.

Pro:

• Indirect memory accesses don't have a zero extension in the address calculation, so the decompiled c code looks better.
• Code can be larger than 32kB.
  Con:
• Constants in flash memory must be within the first 32kB of the flash.
• The stack pointer + 0 problem still exist (bug in Ghidra, not in the CPU definition)