linker: xtensa: move IDT_LIST region

Move the IDT_LIST memory region to the location recommended by
`intlist.ld`. The documentation specifies that this region should not
overlap other regions, and there is no guarantee that the area after the
`SRAM` region is not used. The end of the address space is much less
likely to be a valid RAM address.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>