keystone2: add env option to do unitrd dt fixup

With latest v3.13 kernel, unitrd dt fixup is not needed. However for
older kernel versions such as v3.8/v3.10, it is needed. So to work
with both, add a u-boot env variable that can be set to do dt fixup
for older kernels.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>