ARM: DRA7: Set serial number environment variable

This patch populates serial number environment variable from
die_id_0 and die_id_1 register values for DRA7xx boards.

The function is added in omap common code so that this can be re-used.

Serial# environment variable will be useful to show correct
information in "fastboot devices" commands.

Ref:
http://git.omapzoom.org/?p=repo/u-boot.git;a=commit;h=a6bcaaf67f6e4bcd97808f53d0ceb4b0c04d583c



Signed-off-by: Angela Stegmaier <angelabaker@ti.com>
Signed-off-by: Dileep Katta <dileep.katta@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>