ARC: allow uImage creation for ARC

Starting from U-Boot v2014.04 ARC architecture is supported,
so now it's possible to create uImage for ARC as well.

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>

Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>