arm: am57xx: Set serial# variable

arm: am57xx: Set serial# variable

serial# variable is used to correctly display device ID in
"fastboot devices". It also can be used further for displaying device ID
in "adb devices" (should be passed as "androidboot.serialno" to kernel
cmdline, via "bootargs" variable).

Serial number generating algorithm is described at [1].

[1] http://lists.denx.de/pipermail/u-boot/2015-March/207462.html

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>