stm32f429: pass the device unique ID in DTB

stm32f429: pass the device unique ID in DTB

Read device unique ID and set environment variable "serial#".
Value would then be passed to kernel through DTB.

To read ID from DTB, kernel is required to have commit:
3f599875e5202986b350618a617527ab441bf206 (ARM: 8355/1: arch: Show
the serial number from devicetree in cpuinfo)
This commit is already mainline since v4.1-rc1.

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
To: Albert Aribaud <albert.u.boot@aribaud.net>
To: Tom Rini <trini@konsulko.com>
To: Kamil Lulko <rev13@wp.pl>
Cc: u-boot@lists.denx.de