cmake: linker: dont place device tree memories at absolute addresses

Avoid placing the device tree memories at an absolute address.
This avoids placing both .rom_start AND SSRAM1 at
the same address on mps2/an521/cpu0.

This seems fine according to ld (since one placement is empty)
but causes a currently non-suppressable diag_warning

Signed-off-by: Robin Kastberg <robin.kastberg@iar.com>