🔒 Specifiy Physical Memory Attributes

[stnolting]

This PR specifies Physical Memory Attributes (PMAs) for each of the default address space regions:

#	Region Description	PMAs	Note
1	Instruction address space	r(w)x	Write accesses to the the internal instruction memory (IMEM) can be disabled.
2	Data address space	rwx	Code can also be executed from data memory.
3	Bootloader address space	r-x	Read-only memory.
4	On-Chip Debugger address space	---	Not accessible at all by "normal" software - accessible only when the CPU is in debug-mode.
5	IO/peripheral address space	rw-	Read/write accesses only.