arm: keystone: Move cmd_ddr3 to a common place

Move cmd_ddr3 to cmd/ti in order to make
it build for non-keystone TI platforms.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
[trini: Rename to ddr3.c not cmd_ddr3.c]
Signed-off-by: Tom Rini <trini@konsulko.com>