hw/bsp/lpc55: implement board_get_unique_id from flash PFR UUID

Read the 128-bit device UUID from the flash PFR region at 0x0009FC70
(UM11126 rev 2.1, section 48.8) rather than falling back to the fixed
weak default in hw/bsp/board.c.

Verified on lpcxpresso55s69: cdc_msc enumerates with SerialNumber
E059C3E208F9B955B3BA4C5CC7F3D13D, matching the uid already recorded for
that board in test/hil/local.json.