stm32f4: Set unique ID properly
stm32f4: Set unique ID properly

Unique device ID register (96 bits) is at a different place on STM32F4
compared to other STM32.

BUG=none
BRANCH=none
TEST=Using board_read_serial() from hammer/board.c in
sweetberry/board.c, confirmed that we can extract and assign a unique
USB serial number.

Change-Id: Idb257f0f20422482c729a2b97b4b16ee231ca4d9
Signed-off-by: Gwendal Grignou <190c5a5edfcf62e0246559bf31fa7548674de3ad@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/579575
Commit-Ready: Gwendal Grignou <190c5a5edfcf62e0246559bf31fa7548674de3ad@google.com>
Tested-by: Gwendal Grignou <190c5a5edfcf62e0246559bf31fa7548674de3ad@google.com>
Reviewed-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec