ARCv2: Support dynamic peripheral address space in HS38 rel 3.0 cores

ARCv2: Support dynamic peripheral address space in HS38 rel 3.0 cores

HS release 3.0 provides for even more flexibility in specifying the
volatile address space for mapping peripherals.

With HS 2.1 @start was made flexible / programmable - with HS 3.0 even
@end can be setup (vs. fixed to 0xFFFF_FFFF before).

So add code to reflect that and while at it remove an unused struct
defintion

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>