ARM: keystone2: enable initrd fixup for LPAE addressing

Keystone2 u-boot loads the initrd image into non-LPAE addressed memory
but linux kernel is running in LPAE. This causes a conflict as kernel
detects that non-memory address is passed and kernel ignores initrd.
There is an existing fixup logic to modify the address in the proper
configuration, but this is disabled at the moment. Enable the fixup
by setting the env variable for this so that initrd can be used
properly.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>