arch: Simplify linker script region management using region aliases

Region aliases allow us to specify alternative names for memory regions.
This is particularly useful for sections that are not located in mem0,
as it allows us to conditionally alias the mem1 region based on the
firmware configuration.

Change-Id: I490c513758caa8580fd1d633dcf8f774290b45cc
Signed-off-by: Chris Kay <chris.kay@arm.com>