binman: Fix FIT image overlap issues

binman: Fix FIT image overlap issues

Fix three binman tests that has memory region overlap issue, the test
cases needed to be updated to use non-overlapping memory layouts.

* Tests fixed:
  - testFitFirmwareLoadables
  - testFitSignSimple
  - testFitSignNoSignatureNodes

* Changes made:
  Updated DTB test files to change U-Boot load addresses from 0x0 to
  0x2000 to avoid overlapping with ATF in the 0x10-0xfc range:
  - 276_fit_firmware_loadables.dts
  - 340_fit_signature.dts
  - 342_fit_signature.dts

An upcoming commit will validate if the memory region is overlapped

Signed-off-by: Aristo Chen <aristo.chen@canonical.com>
Reviewed-by: Neha Malcom Francis <n-francis@ti.com>